David Cameron has urged people to vote for their "preferred Prime Minister" as he kicked off a final push to election day.

The Tory leader made a direct appeal for Ukip and Liberal Democrat supporters to cast their ballots tactically and keep him in Downing Street.Mr Cameron insisted he is the "only option" for avoiding a "calamitous" and "chilling" tie-up between Ed Miliband and the SNP.

The plea, in a speech to activists at Ambleside Sports Club in Nuneaton, comes with the polls still showing the Conservatives and Labour effectively deadlocked.

Mr Cameron pleaded for voters to use the long Bank Holiday weekend to "stop and think" about the danger they face.

"It's no exaggeration to say Britain will be making an historic choice," he said. "Ed Miliband wants us to take a sharp turn to the left. He sincerely believes more borrowing and more taxes will help you and your family. He is sincerely wrong.

"Worse - because he will lose so many seats to the SNP in Scotland - he cannot form a majority Government on his own.

"He keeps trying to deny it - but it is a fact that he can only become prime minister with SNP support.

"Nicola Sturgeon and Alex Salmond will extract a heavy price for that support, vote-by-vote in the House of Commons. That price is even more borrowing and taxes to pay for more welfare.

"That is the road to ruin. It would be a calamity for our country, for you and your family."

Hoping to capitalise on his relatively strong personal ratings, Mr Cameron asked those thinking of voting Ukip or Lib Dem to switch to the Tories in order to keep him in Number 10.

"If you have got a view on who you would prefer as your Prime Minister express it at the ballot box," he said.

"The outcome will not be decided any other way.

"Do not risk voting for another party and hoping that is the outcome. If you want your preferred Prime Minister get out there and vote for it."

He warned that Ukip was a "back door to a Labour Government", and suggested Nick Clegg was ready to be part of a "minority Labour Government, propped up by the SNP".

The Conservatives have used their latest election broadcast to press home apocalyptic warnings about the threat of SNP influence.

In the film, the economy, represented by a clock, is smashed by a sledgehammer that represents the nationalists.

Mr Cameron then appears and again brandishes the note Liam Byrne left on his Treasury desk after Labour's 2010 election defeat, which read: "Dear Chief Secretary, I'm afraid there is no money. Kind regards - and good luck! Liam."

The PM adds: "Vote for the Conservatives on Thursday and you will have the security of a plan that is working. You'll have stable government and the strong economy on which our schools, our NHS and your family's future depend.

"But vote for any other party and Britain's recovery could be stopped dead in its tracks."

Leader of the Commons William Hague has compared the position of the SNP with that of Irish nationalist parties in the early 20th century. They won increasing numbers of Westminster seats in the years before Ireland won its independence in 1921 and held the balance of power after the 1910 general election, securing a home rule bill as a result.

"Maybe there are parallels," the former Tory leader and history writer told Sky News' Murnaghan.

"There were a huge number of Irish nationalists elected to Parliament in the 19th century and the early 20th century and that did distort British politics for a long time. It's certainly not good for the UK as a whole.

"What would happen in this situation, if there was a weak Labour government held to ransom by Scottish nationalists is that those nationalists would try to divide Scotland against England with everything they did every day."

Answering questions after his speech, Mr Cameron dismissed the idea that he had not engaged with the public enough during the campaign.

"I think we have taken our message to the country in all sorts of different ways," he said.

"In Wetherby last week I did a sort of open air speech to members of the public as well as supporters.

"My opponent has not done that.

"I have done walkabouts, in places like Alnwick, met people randomly in the streets - my opponent has not done that.

"I have been in factories with audiences, sometimes Conservative voters, sometimes undecided voters, sometimes people who would not dream of voting Conservative.

"I have done it rolled up shirt sleeves, out there in front of people.

"I have done it sometimes with a lectern in front of me. My opponent seems to have a lectern wherever he goes."

Mr Cameron said he thought the BBC Question Time special last week had been a good example of engagement, and said the Tories had also been making full use of social media and Facebook.

Mr Cameron again insisted that an in/out referendum on Britain's EU membership would be a red line for him in any coalition talks if Tories fail to achieve an overall majority. He warned that Britain would "drift towards the exit" if a referendum was not held.

The Prime Minister told the BBC: "What I am saying is that people would worry that were we to fall short - and I don't believe we will - this is something that could be bargained away and I want to be absolutely clear with people that that will not happen ... Come what may, I will not be PM of a government that does not deliver that referendum."

Mr Cameron was repeatedly pressed on whether he has a "Plan B" in case he is unable to secure sufficient reform of the terms of Britain's membership to enable him to recommend a vote to stay in.

"I'm confident we will get the deal," he responded. "I've demonstrated. People said you'll never cut the EU budget, I cut it. People said you'll never veto a treaty, I vetoed a treaty. So people know with me, I have a strong record of negotiating in Europe.

"Plan B is to hold a referendum. And for the British people to choose ... The only thing you have to be clear about is to have a plan for the changes you want, which I've been clear about. The referendum that then takes place, and I've said I'm confident I'll get those changes and I will recommend Britain stays in a reformed European Union and people know that that is my aim, that is my goal.

"But the real point here ... is if you don't have a plan for Europe, Britain will just drift towards the exit. The other parties are sticking their head in the sand thinking this issue will go away. It won't. It's time for some decisive action, some decisive leadership."