The parliamentary standards commissioner has been urged to investigate the conduct of two SNP MPs in relation to their accommodation expenses amid allegations of affairs.
Scottish Conservative deputy leader Jackson Carlaw has written to Kathryn Hudson claiming Angus MacNeil and Stewart Hosie, the SNP's deputy leader, may have "fallen short" of the MPs' rules of conduct.
Both SNP members are alleged to have had affairs with Westminster journalist Serena Cowdy.
Any suggestions of "financial impropriety" in relation to expenses are "totally wrong", the SNP has said.
Mr Carlaw said: "The SNP says there is no financial impropriety - but at the same time it is widely reported that Angus MacNeil stayed with Ms Cowdy at a hotel in London and put the bill on expenses.
"The best course of action is for the UK Parliament's standards commissioner to look into the matter, which is why we are writing to the commissioner this morning calling for an investigation."
The Daily Mail has alleged Ms Cowdy was a frequent guest of Mr MacNeil, 45, at the Park Plaza in Waterloo - a hotel often used by the Western Isles MP.
Mr MacNeil chooses to stay in hotels while working in Westminster and rents out the flat he owns in the capital. He claims the cost of the hotels back in parliamentary expenses.
MPs do not have to disclose details of any guests to the authorities.
An SNP spokeswoman said: "Angus MacNeil's accommodation has nothing to do with it. To suggest any financial impropriety is totally wrong."
Mr Hosie, 53, is also alleged to have had a relationship with Ms Cowdy. He claims expenses on a flat in London.
In his letter to Ms Hudson, Mr Carlaw wrote: "I am writing to ask you to investigate reports that two serving members of parliament used accommodation allowance to conduct affairs with a political journalist.
"The MPs in question are Angus MacNeil and Stewart Hosie, and it has been alleged in several media reports that they conducted their respective affairs in either accommodation or hotels paid for by the taxpayer."
Mr Carlaw suggested the MPs may have "fallen short" of rules which state members "shall ensure that their use of public resources is always in support of their parliamentary duties", and "m embers shall never undertake any action which would cause significant damage to the reputation and integrity of the House of Commons as a whole, or of its members generally".
Mr MacNeil announced his separation from his wife last year while Mr Hosie and wife Shona Robison, Scotland's Health Secretary, announced on Sunday they were separating.
SNP leader Nicola Sturgeon refused to be drawn on the subject, stating it was "a private matter".
A party spokeswoman said: "Stewart Hosie MP and Shona Robison MSP have separated. They have a young daughter and she will continue to be their priority.
"Both parties have asked that their privacy is respected during this difficult time. No further comment will be given."
David Cameron poked fun at the SNP MPs from the Despatch Box in the House of Commons.
In a veiled reference to the allegations, the Prime Minister said: "I know the SNP have got other things on their mind - I think actually it's mostly the same thing on their minds."
Asked about the matter during his LBC radio phone-in, Alex Salmond dismissed Mr Carlaw's call for an expenses investigation.
The SNP MP said: "If it is investigated it will be flung out. There is no expenses issue here whatsoever, none whatsoever.
"The only person who has referred it is a Tory MSP called Jackson Carlaw. I can guarantee you a Tory MP would never refer that because you would have wiped out half the parliamentary Conservative party over the past 20 years ago.
"Look there is no expenses issue here, there is merely the issue of people's private lives.
He added: "I stick to my dictum that I have never commented on people's personal lives and never shall. You know, those who are without sin should cast the first stone."
A SNP spokesman said: "The expenses were approved by the Commons authorities in line with the rules on MPs' London accommodation, and none of them involved any additional cost to the public purse beyond that.
"This is the height of hypocrisy from the Tories, given the extent of flagrant abuse of expenses by their MPs which has been exposed in recent years - they simply don't have a leg to stand on when it comes to this issue."