David Cameron has "full confidence" in Caroline Spelman, the under-fire Conservative chairwoman, the Tory leader's spokesman insisted last night.

Fresh claims about Ms Spelman's pay arrangement with her nanny nine years ago have put pressure on Mr Cameron.

John Lyon, Westminster's Standards Commissioner, is already investigating the arrangement the Midlands MP had with her nanny, Tina Haynes, when she employed her not only to look after her children but also to act as her constituency secretary.

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The arrangement was changed after the matter was drawn to the attention of the party whips, who concluded it could be misconstrued.

However, on Thursday night it emerged that Sally Hammond, who became Ms Spelman's secretary in 1999, was "shocked" at just how much money the Tory chairwoman had been paying Ms Haynes. It was claimed that Ms Hammond found Ms Haynes had done little or no secretarial work and she could not understand why the MP for Meriden had so little money to run her office.

Yesterday, it was reported that senior Conservative MPs had called on party chiefs to sack Ms Spelman from her party post, which involves overseeing probity in the use of MPs' expenses. Michael Portillo, the former Tory Defence Secretary, entered the row by saying he thought it was now time for Mr Cameron to fire his colleague as the controversy was becoming "too murky".