'Hypocrisy' charge as Tory MSP stands for Westminster
Labour said the move was an "embarrassment" to Tory leader David Cameron, who has criticised the "dual mandate" that allows politician to sit in two parliaments.
But the Tories accused Labour of misrepresenting Mr Cameron by omitting his pledge that any Tory so elected would give up the other seat.
The party said Roxburgh and Selkirk MSP Mr Lamont – chosen as Westminster candidate at the weekend – planned to step down as MSP at the 2011 Holyrood election if he became MP.
"Labour have seriously misrepresented David Cameron's position, and deleted a significant paragraph from his speech," said the Tories.
Earlier, Labour candidate Ian Miller said: "This is embarrassing for the Tory leader.
"David Cameron cannot be both for and against the dual mandate at the same time."
The Liberal Democrats' Holyrood chief whip, Mike Rumbles, said: "This is utter hypocrisy from the Scottish Conservatives. (Scottish Tory leader] Annabel Goldie berates three-jobs Salmond in one breath, but urges John Lamont to try for an MP's seat.
"People in Roxburgh and Berwickshire deserve a full-time MSP as much as those in Gordon."