The SNP would block a minority Conservative government by voting down its Queen's Speech if it held the post-election balance of power, its former leader Alex Salmond has said.

Mr Salmond said the move could bring down the government if Labour joined in, with David Cameron "locked out".

The Conservatives accused the ex-SNP leader of "trying to sabotage the democratic will of the British people".

Labour has called his balance of power prediction "bluster and bluff".

In recent days Mr Salmond, who is bidding to become a Westminster MP at the 7 May general election, has claimed his party could "hold the power" in a hung Parliament and would be able to influence the Budget of a minority Labour government.

'Poodle'

This was dismissed by Labour leader Ed Miliband, who has ruled out a formal coalition with the Scottish nationalists.

But that did not stop David Cameron claiming the SNP leader had taken the entire Labour Party hostage and was treating Mr Miliband as his "poodle".

Speaking in the final Prime Minister's Questions before the general election, Mr Cameron hit back at claims he was a "lame duck", telling MPs: "I'll tell you what is a lame duck and that is trying to get into Downing Street on the back of Alex Salmond's coat-tails."

Addressing Mr Miliband across the despatch box, the PM added: "Never mind talk of ducks, I'm looking at Alex Salmond's poodle."

The SNP currently has six MPs at Westminster, but recent opinion polls suggest its share could increase dramatically at the general election.

Analysis

James Cook, BBC News Scotland correspondent

Image copyright PA

If the Conservatives attempt to form a minority government and enough MPs vote against their Queen's Speech the administration would fall at the first hurdle and the leader of the opposition would have a shot at forming a government.

That's how parliamentary democracy works in the event of a hung Parliament.

And yet the Conservatives are saying that by planning to vote against a government he opposes Alex Salmond has "confirmed he would sabotage the democratic will of the British people in order to make Ed Miliband prime minister".

It is part of their continuing attempt to portray Mr Miliband as a weak leader whose strings are being pulled by Mr Salmond.

This kind of language causes despair among Tories north of the border who fear that the party in London is waltzing into an SNP trap.

They know that many voters in Scotland will read the phrase "democratic will of the British people" and hear "democratic will of the English people".

Because, if it is illegitimate for Scottish MPs, from whatever party, to vote against one government and support another in the British parliament, then what, voters may ask, is the point of the union?

Who is dancing to Mr Salmond's tune now?

Mr Salmond stood down as SNP leader after narrowly failing to gain Scottish independence in last September's referendum. Nicola Sturgeon, who took over as SNP leader, has said she will lead any post-election negotiations for the party.

But Mr Salmond, the former Scottish first minister, seems certain to have a role, if he succeeds in his bid to become an MP at Westminster.

'Anti-Conservative rhetoric'

He told the New Statesman that in the event of a hung Parliament, a minority Conservative government would have to "go straight effectively for a vote of confidence - usually the Queen's Speech, although it could be otherwise, of course - and we'd be voting against".

If David Cameron lost this vote, he said, there would be a two-week period for another government to be formed.

BBC political correspondent Iain Watson said Mr Salmond had "hardened his anti-Conservative rhetoric" in the interview.

Commons leader William Hague said: "What is clear now is that you know what you get if you vote Labour - you get a weak Labour government - even if the Labour Party had fewer seats than the Conservatives - propped up by Alex Salmond and at the mercy every day of Alex Salmond."

Asked if the Conservatives were encouraging Mr Salmond to speak out in this way as it suited their agenda, Mr Hague said: "This isn't something manufactured by the Conservative Party, this is something Alex Salmond is saying."

Mr Salmond's latest comments come after Mr Miliband said the only people writing a future Labour Budget would be himself and shadow chancellor Ed Balls.

Speaking in Clydebank on Monday, he added: "It's not going to be Alex Salmond - not in a million years."