Alex Salmond has insisted the SNP could have a "substantial influence " at Westminster after May 7 as he laughed off claims that a minority Labour government propped up by Scottish nationalists would bring about the worst "constitutional crisis" the UK has faced for almost 80 years.

The former SNP leader claimed Tory Home Secretary Theresa May was "overwrought" after she suggested an alliance between the SNP and Ed Miliband would be ''the biggest constitutional crisis since the abdication'', when King Edward VIII gave up the throne in 1936.

The former Scottish first minister said: "I think the best thing to do is to laugh at this Tory campaign."

The Conservatives have repeatedly warned of the risks the SNP could pose to the UK if the party, which is tipped to win as many as 50 seats, is left holding the balance of power after the election.

David Cameron has likened Mr Salmond to a pickpocket, and a new Tory billboard advert shows him dressed all in black, reaching out to steal the wallet of a passer-by.

Mr Salmond, who stood down as first minister and SNP leader after Scots voted to stay in the UK last year, quipped that in the poster he was "debonair, dashing, dressed in black".

He told the Murnaghan programme on Sky News that a substantial group of SNP MPs at Westminster would be good not just for Scotland but for "progressive politics" across the UK.

Mr Salmond said: "The best thing to do, and this applies not just in Scotland but right across the country, is to start laughing at this Conservative campaign.

"The people of Scotland understand it's going to be a good thing for Scotland to have a strong group of Scottish National Party MPs.

"But as Nicola Sturgeon has been brilliantly laying out in this campaign it's going to be good thing for those who believe in progressive politics across the UK, people who want an increase rather than a decrease in public spending, people who care about the health service, the education system right across these islands.

"That's why the SNP campaign has such momentum, because we're putting forward a positive, inspiring vision, as opposed to this nonsense from the Conservative Party and a pretty negative campaign from the Labour Party as well."

He stated: "This stuff from Theresa May about the worst constitutional crisis since 1936 - has she forgotten the Second World War or more recently the illegal war in Iraq, which the Labour Party and the Conservatives dragged this country into?

"For goodness sake, the woman is overwrought, as indeed is David Cameron.

"We've got a Prime Minister who can't even remember the name of his football team and a Home Secretary who is becoming a social media parody, there's whole sites going up making fun of Theresa May's panic and desperation."

With less than two weeks to go until the election, Mr Salmond said: "We're in an interesting situation where the stars are coming into alignment and a group of Scottish MPs, of SNP MPs, could have a substantial influence at the House of Commons. Unfortunately that doesn't happen very often."

But he stressed the vote on May 7 was not a re-run of the independence referendum, saying: "The Westminster election is about getting more power and influence for Scotland and helping people across the UK in terms of progressive politics, that's what this election is about.

"You have to have a situation in a future Scottish election where a majority of people voted for a party or parties that had pledged to hold an independence referendum. That is why it is the people of Scotland who will decide if and when there is another independence referendum."