First Minister Alex Salmond should quit as an MP as his voting record shows he treats constituents "with contempt", say the Scottish Conservatives.
Mr Salmond holds seats in both the Scottish and UK Parliaments
Mr Salmond is a member of both the Scottish and UK Parliaments and says he will remain so until the next election.
The Tories point out that since the SNP won power last May, he has taken part in four of 149 Westminster votes.
But the SNP said its MPs only took part in votes on Scottish issues and called the Conservatives' claims "desperate".
Mr Salmond has been MP for Banff and Buchan since 1987 and was elected to the Scottish Parliament in May last year, as MSP for Gordon, as well as becoming First Minister of Scotland.
He has previously faced calls from Labour and the Lib Dems to resign his Westminster seat - but Mr Salmond has pointed out that other MSPs have served as MPs, including former deputy first minister Jim Wallace and former first minister Donald Dewar.
Deputy leader of the Scottish Conservatives Murdo Fraser told the BBC he had warned that if Mr Salmond became MSP for Gordon, the constituency would be getting a part-time MP until the next Westminster election, which could be as late as 2010.
He said: "The current situation will mean that, for up to three years, Alex Salmond will be doing three jobs at once.
"He simply is not doing the job of Member of Parliament for Banff and Buchan. It's inconceivable that he can carry on like this.
"Alex Salmond needs to step down, so there can be a by-election and Banff and Buchan can have a full-time MP again.
"To do otherwise is treating his constituency with contempt."
But an SNP spokesman told the BBC it was a "desperate" move from the Conservatives - and said Mr Salmond had made it clear to voters in both his constituencies that he would stand for both jobs.
Despite this, he achieved his second-best ever result in Banff and Buchan in 2005 and an 11% swing from the Lib Dems in Gordon in 2007, he said.
"I think by those measures we can safely say that the people of north-east Scotland have spoken and are more than content with the arrangement," he said.
The party argues that Mr Salmond's role as an MP allowed him to make important contributions in debates following terrorist attacks on Glasgow and London last year.
The spokesman added: "There is much more to being an MP than trooping the lobbies at Westminster, as is shown in the latest available league table for MPs' participation.
"Encompassing speeches in the chamber, tabling of motions and submitting written and oral questions, the analysis shows all six SNP MPs, including Mr Salmond, are in the top 10 of Scottish MPs."
Mr Salmond has said he will step down as an MP at the next general election. The SNP says it has already chosen its candidate for Banff and Buchan.