Legislation passed under the last government would give the Scottish National party "huge influence" in a hung parliament and Westminster politicians have not fully grasped the implications, according to former SNP leader Alex Salmond.
In an interview with the Financial Times just days ahead of an expected nationalist surge north of the border, Mr Salmond said many in the House of Commons appeared not to have read the 2011 Fixed-term Parliaments Act.
"It's only votes of confidence that can bring down the government . . . and you can vote as you wish on everything else," he said. "There are many things that the Labour party would want to bring forward which we would have to be persuaded to support."
Under the fixed-term law, an early general election can only be called if a two-thirds majority of the House of Commons votes for it or if a vote of no-confidence in the government is carried and 14 days passes without an alternative administration winning a vote of confidence.
Mr Salmond, who is standing for parliament in the Aberdeenshire constituency of Gordon, said this meant the SNP could potentially force a Labour minority leader to compromise even on the Budget. This is because defeat on such a vote would not bring down the government and the prime minister could simply come back with a new version to win a majority.
As an example, he noted the budget defeat suffered by his own minority government in Scotland in 2009. The SNP won passage for its budget a few weeks later after concessions to opposition parties.
"In a balanced parliament, a strong group of SNP MPs, with our allies in the Green party and Plaid Cymru, will have huge influence regardless of the complexion of the government," Mr Salmond said.
Potential SNP influence in Westminster is a topic of fierce debate, with the Conservatives claiming the Nationalists could hold a Labour leader to ransom if, as expected, they win most of Scotland's 59 seats on Thursday. Plaid Cymru had three seats in the last parliament and the Greens, one.
Ed Miliband, Labour leader, has said he would rather not be in power than do a deal with the SNP and some of his colleagues say the Nationalists' pledge not to put the Tories into power would leave them little leverage.
Mr Salmond, who resigned as SNP leader after losing last year's independence referendum, said such arguments misunderstood the 2011 act. "I have taken the precaution of reading the Fixed-term Parliaments Act," he said. "It only surprises me from some of their comments that others don't seem to have bothered, even though they actually passed the legislation."
Some in Westminster say Westminster's culture is very different to that of Holyrood's and that the defeat of a Budget would force the resignation of a Labour prime minister.
But Mr Salmond, who has sat in the House of Commons or the Scottish parliament continuously since 1987, said he had "more parliamentary experience than Ed Miliband and David Cameron put together".
"I think I kind of know how Westminster works," he said. "They will have to knuckle down and get on with it . . . we had to make concessions, and they will have to make concessions."
The SNP could vote with the Tories against Labour as long as it had public support on the issue at hand, he said.
While Mr Salmond stressed the influence the SNP could gain, vote-by-vote dealing would also mean his party would have little chance to shape policy on issues such as more power for Scotland or replacement of Trident nuclear weapons, where Labour could draw on Conservative support.
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