The home secretary says her sister's home in London is her main residence
Parliament's sleaze watchdog is facing calls to launch a full investigation into Home Secretary Jacqui Smith's use of MPs' second home allowances.
John Lyon has asked Ms Smith to justify designating her sister's London home, where she stays when in the city, as her main residence.
It follows a complaint - firmly denied by Ms Smith - that she spends as little as two nights a week there.
But former MP Martin Bell has written to Mr Lyon to demand a formal probe.
In his letter to the parliamentary standards commissioner, Mr Bell claims there is clear evidence that the rules have been broken.
The former Tatton MP, who defeated disgraced Conservative former minister Neil Hamilton in 1997 on an anti-sleaze ticket, says: "It is my understanding that the commissioner is required to investigate any complaint where there is any evidence that a rule has been broken.
"Clear prima facie evidence exists in this case - indeed it is growing stronger by the day."
Ms Smith's neighbours, Dominic and Jessica Taplin, who made the complaint to the standards commissioner, allege the home secretary spends an average of three nights a week at the London address.
Mr Bell says this proves that "the home which Ms Smith claims as her main residence is not where her family live or even where she spends most of her time".
"It cannot realistically or in a common sense view be regarded as anything other than a secondary home.
"This expenses arrangement is having the effect of bringing the House of Commons yet again into disrepute. Given the terms under which you operate, I am sure that you would not wish this to happen on your watch.
"Because of the importance of this case I would urge you to reconsider the complaint against the MP for Redditch, and indeed to regard this letter as a fresh complaint against her. If you decide not to reconsider, I would suggest that there is a strong public interest for you to explain the reasons for this decision."
Ms Smith's decision to call her sister's home her main residence has enabled her to claim at least £116,000 in second home allowances on her constituency home in Redditch, Worcestershire, since becoming an MP.
She has consistently denied any wrongdoing, insisting that she has written approval from the Commons fees office for her arrangements.
Sources close to the home secretary have told BBC News she spends the bulk of her time in London but had asked the police to maintain a "low profile" so the neighbours may not have realised she was in residence.
They say Ms Smith's children often stay with her in London but add that the Commons fees office had told her it was "irrelevant" for allowances purposes where her family lived.
Ms Smith told BBC News: "I followed the advice I was given and I have followed the rules" but added that she would "respond to any questions the independent commissioner asks me".
Mr Lyon has asked Ms Smith to explain how much time she spends at the south London address but has yet to launch a formal investigation into her conduct.
Meanwhile, Sir Alistair Graham, former chairman of the Committee on Standards in Public Life, set up in 1994 in the wake of the "cash-for-questions" scandal, said the Jacqui Smith affair reflected badly on the government and on MPs in general.
He said there needed to be reforms to the system of allowances which would mean MPs "may get less allowance money but their standing in the community will be very much better".
He told the Evening Standard Ms Smith was failing to show moral "leadership" by apparently maximising her expenses at a time of recession.