Labour could take a critical blow in Scotland at the general election as the party's campaign manager and shadow foreign secretary faces losing his seat.

Douglas Alexander, who represents Paisley and Renfrewshire South, would lose his place in the House of Commons with a 25% swing to the Scottish National Party (SNP), according to Lord Ashcroft.

The former Tory party deputy chairman, who surveyed more than 16,000 people, found that respondents' standard voting intention was 39% for Labour and 45% for the SNP, and their constituency voting intention was 40% for Labour and 48% for the SNP.

The long-awaited polling also revealed that former First Minister of Scotland Alex Salmond is set to unseat the Liberal Democrats in Gordon and return to Westminster, with a swing of 15.5% to the SNP in the constituency.

"Most of my constituency research is focused on marginal seats. But in post-referendum Scotland, the concept of a marginal seat is rather obsolete," Ashcroft said.

"Huge swings to the SNP in national polls suggest that even some MPs who must have thought they had a job for life are threatened.

"My first round of Scottish constituency research therefore required a different approach. I decided to look primarily at Labour seats – including some with colossal majorities – in areas which voted yes to independence, or where the result was very close."

In addition, the polling found that the Liberal Democrat chief secretary Danny Alexander, who has been the Member of Parliament for the Inverness, Nairn, Badenoch & Strathspey constituency since 2005, would lose by 29 points at the general election.

The research also revealed that, in the Labour-held seats, only 60% of those who voted Labour in 2010 said they would do so again this year; more than one third (35%) said they would support the SNP.

Lord Ashcroft said while the Conservative vote has held up in these seats well, the Liberal Democrats have "collapsed": only 12% of the party's 2010 supporters said they would vote Liberal Democrat again; nearly half (47%) said they would switch to the SNP.

The findings come with less than 100 days to go before the general election and as the political parties ramp up their campaigning.