David Cameron faced calls to sack his frontbencher after it emerged her own secretary had first raised concerns that she was paying a nanny with taxpayers' money.

BBC2's Newsnight revealed that Sally Hammond - who became a part-time secretary to Mrs Spelman in 1999 - was "shocked" to discover just how she much was paying to Tina Haynes.

Ms Hayes was employed by Mrs Spelman when she became an MP in 1997 to look after her children and do secretarial work. But Mrs Hammond found that Ms Haynes did little or no secretarial work to justify the payments.

Mrs Hammond - the wife of Tory frontbencher Stephen Hammond - was said to have become aware of the situation because she could not understand why Mrs Spelman had so little money available for office expenditure.

She took her concerns to shadow cabinet member Peter Ainsworth - who she had worked for in the past - who in turn took it up with Chief Whip James Arbuthnot and the arrangement was stopped.

A number of Tory MPs are now reported to have contacted the chairman of the backbench 1922 Committee, Sir Michael Spicer, demanding Mrs Spelman should be sacked.

A party spokesman said on Thursday night that Mrs Spelman was co-operating fully with the inquiry into the matter by Parliamentary Standards Commissioner John Lyon. "Caroline Spelman asked for the inquiry herself and will happily provide all relevant information to the parliamentary commissioner," the spokesman said.

But Labour MP John Mann today wrote to Mr Cameron demanding that he take "prompt" action in the face of strong evidence.

"I trust that it is not your intention to discretely dispose of her during a reshuffle and that you will ensure that all the material in your Party's hands is given to Mr Lyon," he insisted.