The Conservatives are calling for an investigation into claims that Cherie Blair used her influence to help an acquaintance lobby for NHS contracts.
Mrs Blair is with her husband at the G8 summit in St Petersburg
The Mail on Sunday said Mrs Blair helped Jonathan Metliss, director of a health firm, set up a meeting with the prime minister's health adviser.
Mr Metliss was the boyfriend of a close friend of Mrs Blair, the paper claimed.
Downing Street said there had been a "misunderstanding" and that the adviser set up the meeting himself.
Paul Corrigan, Tony Blair's senior health adviser, met Jonathan Metliss and Harry Hyman, of the Nexus Group, at a Westminster restaurant last month.
The healthcare firm already has an £8m contract to run an NHS walk-in centre but is bidding to open more.
The newspaper claimed Mrs Blair had lobbied Professor Corrigan to arrange the meeting.
'No action taken'
Shadow transport secretary Chris Grayling said he would write to Cabinet Secretary Sir Gus O'Donnell on Monday asking him to investigate.
"A member of the prime minister's family should not be using civil servants for any purpose.
"They are there to serve the government, not Mr Blair's family."
But Downing Street said in a statement that there had been a "misunderstanding".
"Paul Corrigan took no action following the meeting and did not offer to put them in contact with anyone," the statement said.
Prof Corrigan would meet "dozens of businessmen involved in health care in the course of his working week" the statement claimed.
"There appears to have been a misunderstanding. In fact Mrs Blair has never met Mr Hyman and it was Paul Corrigan who asked his secretary to set up the meeting."