NICOLA Sturgeon and Ed Miliband are "already halfway down the aisle" to forming a new government after the general election, the Scottish Tory leader has warned.

Ruth Davidson kicked off her election campaign by claiming a pact between Labour and the SNP at Westminster could result in Britain being "run into the ground".

With four months to go till the election, party leaders have been rallying their supporters ahead of the campaign.

The SNP, which is ahead in the polls, is seeking to increase its representation at Westminster, with the aim of potentially holding the balance of power if there is a hung parliament.

Nationalist leader and First Minister Nicola Sturgeon has already ruled out supporting the Conservatives if they fail to win a majority, but has suggested her party could do a deal with Labour, if it meets key demands.

In a letter to Conservative supporters Ms Davidson said the First Minister was "talking up an electoral pact between Labour and the SNP, with Alex Salmond heading back to London to seal the deal".

The Scottish Tory leader, whose party currently has just one MP north of the border, stated: "The election is another four months off, and yet Nicola Sturgeon and Ed Miliband are already half way down the aisle.

"Think what it would mean - a weak Ed Miliband as Prime Minister with the SNP pulling the strings. Just when families and businesses need some security, the Nationalists would plunge us back into an uncertain world."

She insisted: "I don't want to see Britain being run into the ground by a Labour-SNP pact."

Ms Davidson said the referendum last year had resulted in a "decisive vote to keep Scotland part of the United Kingdom" but added: "I for one didn't fight to secure our future last year just to see Ed Miliband or Nicola Sturgeon throw it all away."

She argued: "The truth is that the Scottish Conservatives are the only option in Scotland if you want to keep the UK together and keep Ed Miliband out of Downing Street."

Ms Sturgeon has already called on those who voted against independence but want a stronger Scottish Parliament to back the SNP at the general election.

The party has launched a new campaign poster showing tartan seats in the House of Commons, which states: ''The more seats we have here, the more powers we'll have in Scotland.''