Ian Blackford has been urged to give a “full-time commitment to constituents” and quit two second jobs worth around £50,000 a year.

The SNP Westminster leader pocketed the sum for his work as chairman of a funeral plans fund and telecom firm, in addition to his £74,962 MP salary this year.

Election rival Craig Harrow, who is contesting Mr Blackford’s Ross, Skye and Lochaber seat for the Liberal Democrats, has now written to Nicola Sturgeon calling for all SNP candidates to quit their second jobs.

In the letter, seen by the Press and Journal, Mr Harrow said: “As leader of the SNP, I am writing to request you to confirm that SNP candidates at the general election will keep to the vow made by Peter Wishart in 2015 that for MPs ‘there should be no second jobs’.

“I noted that recently, the former SNP Westminster leader Angus Robertson stated that MSPs should ‘give a full-time commitment and not claim they can be in two places, serving two masters at the same time’.

“Will all SNP candidates, including the SNP’s current Westminster leader, therefore pledge a ‘full time commitment to constituents and constituency’ ahead of the election on December 12?”

An SNP source hit back however, saying that several senior Lib Dems had profited from second jobs.

The source said: “SNP MPs work hard full time in their roles standing up for the people of Scotland at Westminster.

“Unlike the Lib Dems, who imposed devastating austerity cuts on Scotland’s communities in coalition with the Tories, SNP MPs always stand up for Scotland’s interests.

“Hopefully after this cringeworthy boomerang attack, Mr Harrow will redirect his letter to Lib Dem leader Jo Swinson, Lib Dem deputy leader Ed Davey, Scottish Lib Dem Jamie Stone, Chuka Umunna, Vince Cable, Tim Farron, Norman Lamb, Philip Lee and other senior Lib Dem spokespeople who have made thousands of pounds in second jobs during their time as MPs.”

He added: “All SNP MPs elected in December will have a full time commitment to their constituents and constituencies.”