A former coalition minister has accused the Conservatives of a deliberate ploy to help SNP MPs get elected in place of Liberal Democrats.

Jo Swinson, the former Lib Dem business minister, said that despite a controversial advertising campaign against the SNP in England the Tory "preference" was for the nationalists to beat her party north of the border.

The Conservatives had ploughed money into no-hope seats in Scotland in a bid to split the pro-Union vote and ensure that the SNP won, she suggested.

In the run-up to the General Election Ed Miliband ruled out relying on a coalition with the SNP to help him into Downing Street.

However, the then Labour leader left the door open to a possible deal with the Lib Dems.

Senior Conservatives are expected to claim that they are the only true pro-Union party left in Scotland at their annual party conference, which opens in Manchester this weekend.

In recent weeks both the Scottish LibDem and Scottish Labour leaders have suggested they would allow their MPs and MSPs to campaign for a Yes vote in a second referendum.

But Ms Swinson said that the Conservatives were "quite content with having an SNP MP rather than a Lib Dem MP".

After the independence referendum polls showed that her main opponent in her East Dunbartonshire seat was the SNP, she said.

She added: “The Conservatives (eventually) came fourth in East Dunbartonshire, with 9 per cent of the vote.

"They were never going to win in East Dunbartonshire. And they knew that.

"But they spent large amounts of money, buying full-page advertisements in magazines that went to every house in half the constituency, having bar charts suggesting that they were the only ones that could beat the SNP.”

She added: “I think they there is an interesting question mark about the wider Conservative strategy.

"In (the party) being quite content with having an SNP MP, in some cases, elected in preference over a Liberal Democrat MP.

“And there were certainly other seats in Scotland where they did the same thing,” she said during a fringe event at the Lib Dem conference in Bournemouth last week.

Senior Lib Dems have previously hit out at Tory adverts about the SNP ‘threat’ at the General Election.

One notorious poster portrayed former party leader Alex Salmond as a pickpocket.

The Lib Dems have also criticised David Cameron’s plan to restrict Scottish MPs' voting rights, announced just hours after the result of the independence vote.

The Scottish Conservative leader will address her party’s conference on Wednesday, just before Mr Cameron’s own keynote speech.

Tory activists have already been warned not to wear their passes in public during the conference, amid fears of unrest and potentially violent clashes.

A number of anti-austerity protests, including one at which Labour leader Jeremy Corbyn is due to speak, will take place in the city to coincide with the Tory event.

Mr Cameron will also come under intense pressure inside the conference centre from the party faithful and his own MPs.

The issue of Europe is expected to dominate with some eurosceptics already calling for Mr Cameron to set a date for his pledged EU referendum, promise government funds will not be used to win it and to allow ministers to campaign against government policy.

A new poll has found most people believe they should be free to argue for the UK to leave the EU.

The Survation survey for the Huffington Post website found that 55 per cent think that ministers should be allowed to lobby for a ‘Brexit’ while just 25 per cent think they have a duty to stick to the Government line.

It is understood that the theme of much of the Conservative conference will be on “security”, including economic security.

A spokesman for the Scottish Conservatives said: “We were only interested in getting Scottish Conservatives elected in May, and were the only pro-union not to promote tactical voting.

“It’s clear the tactical voting ploy so loved by Labour and the Liberal Democrats didn’t work.

“If Ms Swinson is suggesting other parties should have stood back and waved her through that would be incredibly undemocratic.”