Ed Miliband has accused David Cameron of "threatening the integrity of the United Kingdom" by talking up the prospects of Scottish nationalists in the May 7 General Election.

The Conservatives were stepping up their campaign on the dangers of nationalist influence in the new Parliament by deploying former prime minister Sir John Major to deliver a stark warning that a minority Labour government propped up by SNP votes would be a "recipe for mayhem".

With 16 days to polling on May 7, Sir John will say that a Labour-SNP administration would lead to "weak and unstable" government and wreck Britain's economic recovery.

He will use a speech in the Midlands to paint a picture of an Ed Miliband government subjected to a "daily dose of blackmail" by the nationalists who could bring him down whenever they wanted.

But Mr Miliband insisted there would be no coalition with the SNP and denied Nicola Sturgeon's party would hold the whip hand on policy, telling BBC1's Breakfast: "A Labour government led by me, what happens in that Labour government will be decided by me, not by the SNP."

Labour had "fundamental differences" with the SNP, such as the nationalist party's desire for a second independence referendum within five years, he said, adding: "I'm not having that."

Mr Miliband said Mr Cameron had been "talking up" the SNP in the hope that it would take votes and seats from Labour north of the border and allow him to "crawl back" into 10 Downing Street.

"I think David Cameron is playing fast and loose with the United Kingdom," said the Labour leader. "This is somebody who has given up hope of winning a majority. He is trying to boost the SNP.

"I think David Cameron is now threatening the integrity of the UK with the games he is playing. And I think Conservatives are now ashamed of what he is doing."

Mr Miliband cited Conservative former Cabinet minister Lord Forsyth who has warned that the party's tactic of targeting a Labour-SNP link-up was "short-term and dangerous" and could ultimately damage the Union.

In a BBC interview last night, the Labour leader insisted he would not be dictated to by the nationalists, even if he had to govern without an overall majority in the new parliament, saying: "That ain't gonna happen."

But Sir John will argue that, in practice, Mr Miliband would be forced to accede to the SNP's demands or face the collapse of his government.

"If Labour were to accept an offer of support from the SNP, it could put the country on course to a government held to ransom on a vote-by-vote basis," he is expected to say.

"Labour would be in hock to a party that - slowly but surely - will push them ever further to the left. And who would pay the price for this? We all would. We would all pay for the SNP's ransom in our daily lives - through higher taxes, fewer jobs, and more and more debt.

"This is a recipe for mayhem. At the very moment our country needs a strong and stable government, we risk a weak and unstable one - pushed to the left by its allies, and open to a daily dose of political blackmail."

Launching her party's manifesto in Edinburgh yesterday, SNP leader Nicola Sturgeon sought to allay the fears of English voters, insisting they would act "responsibly and constructively" in the interests of the whole of the UK.

But Sir John will say that the SNP's "driving ambition" is an independent Scotland and it would inevitably use its position to demand policies that favour Scotland at the expense - "quite literally" - of the rest of the UK.

"That is no way to run a country. And nor is it remotely fair to England, Wales and Northern Ireland," he will say.

The former premier will point to the way that Scottish Labour leader Jim Murphy has already suggested that funds raised from the party's planned "mansion tax" in England could go to Scotland.

"If a Labour leader asks for that, how much more will the SNP demand?" Sir John is expected to say.

"And if this is the way Labour intends to behave towards England, how can they say no to the SNP? And if Labour did say no, the SNP could withdraw support and bring down the government at any time."

With the opinion polls pointing to a hung parliament with the SNP holding the balance of power as the third largest party, the threat of a nationalist link-up with Labour has emerged as the main Conservative line of attack in the election campaign.

The tactic has caused misgivings among some Tories. Lord Forsyth - who served as Scottish secretary in Sir John's government - warned that building up the SNP to undermine support for Labour in England could ultimately damage the Union.

"We've had the dilemma for Conservatives, which is they want to be the largest party at Westminster and therefore some see the fact that the nationalists are going to take seats in Scotland will be helpful," he told the Guardian.

"But that is a short-term and dangerous view which threatens the integrity of our country."

He said Mr Cameron's call for "English votes for English laws" in the aftermath of last year's Scottish independence referendum vote had "shattered the unionist alliance against the break-up of the United Kingdom".

Ms Sturgeon said that she - rather than her predecessor, Alex Salmond, or the SNP's leader in Westminster, Angus Robertson - would lead any negotiations after an inconclusive election result, despite not standing for a seat in the House of Commons.

She told the Daily Mail: "I'll be there. I'm the leader of the SNP, I'll be in charge of any decisions and negotiations and anything that happens after May 7."

Ms Sturgeon also indicated that a second independence referendum for Scotland could be triggered if the UK votes to leave the EU in 2017.

She said a "substantive change in circumstances" would be needed to re-run the poll, adding: "If there was a vote that ended up taking Scotland out (of the EU) against our will - that's the only concrete example."

Meanwhile, former Labour minister and London mayoral contender David Lammy suggested that the party could "do business" with the SNP after the election.

"I think we can win this election. But clearly, after the General Election, you would forge common alliance with parties that you can actually do business with and the SNP must be part of that story," he told ITV News.

"I still think Labour can form the next government and that's what I am fighting for. But, yes, there is common ground with other parties and the SNP would be included in that and we may need to enter into discussion after the General Election."

Former Liberal leader Lord Steel said Sir John did not understand Scotland.

He told the BBC Radio 4 Today programme: "The Tories are really doing the SNP a favour by bringing them to the centre stage of the campaign.

"I think the consequence is we will end up with a minority government. Ed Miliband has made it clear he does not want any deal with the SNP."

Labour's former chancellor Alistair Darling, who led the No campaign against Scottish independence, said he expected the party to try to form a minority government if it falls short of an absolute majority.

Mr Darling told BBC Radio 4's Today programme: "The idea that we would enter in to any agreement with a nationalist party that has as its sole aim the destruction of the UK - they've no interest in supporting a stable British government - is for the birds. It just won't happen.

"The idea that we would enter in to an agreement that would be destructive not just for the county, but also actually for the Labour Party, is absolute nonsense, so we won't do it."

Mr Darling said that the Conservatives were "entering into a dangerous, destructive embrace of the nationalists" which could lead to "even more division, another divisive period where we argue about the constitution of the country, which actually most people in Scotland don't want". He said the SNP would "struggle" to win a second referendum if it was held now.

He added: "I really do think that the Tories flirting with English nationalism and playing that off against Scottish nationalism is destructive, and it's pretty desperate to put at risk the United Kingdom simply to grab another five years in power."

Mr Darling questioned the idea that the SNP would have a decisive influence if they hold the balance of power: "What are the nationalists going to do if they don't like something that the Labour government is going to do? Are they really going to vote us out and put in the Tories? People with long memories will remember that's what they did in 1979 and they paid a very heavy price for it.

"Whether we have a majority or we are a minority government, we must do what is best for the national interest for every part of the United Kingdom - Scotland, England, Wales and Northern Ireland - but we cannot allow ourselves to be held to ransom by a party whose sole aim is not just the destruction of our party but more seriously the destruction of the United Kingdom."

Ukip economic spokesman Patrick O'Flynn insisted only his party would stand up for the interests of English and Welsh people.

Speaking at a briefing on small business policy in central London, he added: "The thing about the SNP issue is that the leaders of all the other three major parties have signed a vow continuing the Barnett formula.

"None of them are guaranteeing English votes for English laws at every stage of the legislative process.

"So only Ukip will stand up for the interests of English and indeed Welsh voters in the House of Commons after the election."

He also criticised the Prime Minister, suggesting he could be the "fundamental problem" at the heart of the Tory campaign.

"I think it is quite interesting too that David Cameron last week was channelling Margaret Thatcher with half-baked revivals of things like right to buy or share sell-offs," he said.

"This week it's John Major, who let's not forget presided over the economic chaos of the exchange rate mechanism experiment.

"Some observers are saying that the Conservative campaign is in a lot of trouble. If that's the case, then perhaps the fundamental problem at the heart of their operation is that they have a leader who doesn't actually really know what - to use that quote - his irreducible core is. What are his values, what does he stand for?

"I think it is much easier to tell what our leader, what Nigel Farage stands for."

Lib Dem leader Nick Clegg said Lord Forsyth's comments were a sign of the panic in the Tory campaign.

Speaking in St Ives, Cornwall, he said: "The Conservatives are now - and I thought this would happen around this time of the campaign - starting to argue amongst themselves because they are panicking.

"It is now dawning on the Conservatives, something I could have told them ages ago, that they are not going to win this election.

"Everybody knows they are not going to win the election, in fact everybody knows that no one is going to win the election outright, and they are starting to panic. They are thrashing around, using ever more intemperate language."

Democratic Unionist Party deputy leader Nigel Dodds said he expected the party would win eight to 10 seats on May 7, and was ready to talk with Labour and Conservatives with the aim of supporting a "stable" government which excluded the SNP from a position of influence.

Mr Dodds told BBC Radio 4's Today programme: "We want, in the next Parliament, to be important players in strengthening the UK against the tide of nationalism.

"We have made it very clear that we don't seek coalition, we don't seek positions for ourselves. We're not interested in that. We are interested in creating stable government for the United Kingdom. We are very worried indeed by the potential threat in terms of the break-up of the UK by the SNP's potential grip on power.

"We are not ruling out working with either the Labour Party or the Conservative Party. We certainly wouldn't be in coalition and we certainly wouldn't be part of any formal arrangement which would include the SNP."

Conservative Leader of the Commons William Hague denied Tories were "talking up" the SNP.

Mr Hague told BBC1's Breakfast: "I think John Major is absolutely right to say what he is going to say in his speech today and to point to the danger of people who want to break up the United Kingdom in effect running the United Kingdom, if they hold the balance of power in Parliament.

"That's not talking up the Scottish National Party. Voters across the United Kingdom have to be aware of how serious this situation is, how serious this threat is, that it could actually happen and that the SNP are intending to call the tune, as they have said, on a whole range of matters across the UK. That would mean higher taxes, it would mean higher welfare bills, it would mean weaker defences.

"We are not talking them up. Those are the facts on the ground in Scotland at the moment."