Officials were dismayed and stunned as Mrs Blair was forced to confirm that she had called the lawyers to reassure Carole Caplin, Mr Foster’s girlfriend and her own close confidante, that the proceedings were being properly handled.

The disclosure that the Prime Minister’s wife had involved herself in the case against the man who, only days earlier, was helping her to negotiate the purchase of two flats in Bristol was the most damaging yet to hit Mrs Blair, the Downing Street machine and the Prime Minister.

A statement from Mr Foster’s solicitors was the first No 10’s media team knew of the call, and left them wondering where the next “thunderbolt” would come from. It also raised fresh accusations that No 10 has not told the whole truth. Mr Blair’s spokesman said last Thursday that Mrs Blair had not assisted Mr Foster’s legal battle against deportation, but he gave no information about the telephone call.

Last night in a Downing Street statement, Mrs Blair insisted that she had not interfered in the immigration case and had made the call “as a friend of Carole Caplin to reassure Ms Caplin that the solicitors were handling the case in the normal way”.

The Prime Minister, drawn to comment on the affair for the first time last night, told Iain Duncan Smith in a letter that there had been no political interference in the case against Mr Foster.

In an interview with the Financial Times, given before yesterday’s disclosures, Mr Blair tried to dismiss the episode as a media frenzy and suggested that it would be forgotten without lasting damage. “This is just part of what comes with the territory nowadays,” he said. “This type of media frenzy will come and it will go.”

Mrs Blair telephoned the law firm Janes on November 22, eight days after she had sent Mr Foster an e-mail saying: “We certainly are on the same wavelength.”

He has been fighting deportation since August when the Home Office decided, in view of his criminal record, “that his removal would be conducive to the public good”.

Mrs Blair made a conference telephone call to Janes “in respect of his immigraton status and in defence of any alleged criminal conduct”. Ms Caplin was also on the line.

“The avowed and plain purpose of the telephone call was to reassure Carole that the immigration proceedings against Peter Foster were being conducted on a regular and normal basis and there was nothing untoward. We were happy to confirm this,” Janes said. “We wish to emphasise that Cherie Booth, QC, did not intrude into our conduct of the proceedings and for the avoidance of doubt, had no say whatsoever in our choice of representation of counsel.

“In our opinion she was simply seeking to provide support and assurance to her friend Carole and acted with complete propriety.” The statement shocked Downing Street. Only last Wednesday it was taken by suprise when it was revealed that, contrary to its earlier claims, Mr Foster had helped Mrs Blair over the purchase of the flats.

Mrs Blair printed out all her e-mails in relation to the transaction and apologised if she had damaged relations between the media and No 10.

It appears that she did not tell No 10 about her phone call on behalf of Ms Caplin.

A senior aide summed up the despair in No 10 when he said: “It’s drip, drip, drip, drip.” Downing Street issued a statement in response to Mr Foster’s solicitors’ intervention.

It said: “Mrs Blair stands emphatically by her statement that, had she known the details of Peter Foster’s past, she would have been more circumspect. At no point did she interfere in the immigration case proceedings. Nor would she.”

Mrs Blair gave no details of her assistance for Mr Foster in the statement she issued last week after Downing Street denied that he had helped the Blairs to buy a flat.

The Prime Minister’s wife claimed that “in late October” she “was unaware of the details of his past which has since become public. Clearly had I been aware I would have been far more circumspect in my response to what appeared to be straightforward, friendly offers of assistance from the boyfriend of a friend.”

Mrs Blair’s telephone call came after Mr Foster’s original request for a judicial review of his expulsion was rejected by Mr Justice Maurice Kay on October 30. Mrs Blair’s legal advice is reported to cost £250 an hour.

On the day of the conference call, Mr Foster’s solicitors contacted Mrs Blair’s chambers, Matrix, to inquire whether Heather Rogers, a senior barrister, would take the case.

Ms Rogers said she was too busy but the fact that the calls took place on the same day suggests the Prime Minister’s wife was doing more than merely acting as a supportive by-stander.

Six days after Mrs Blair intervened, Mr Foster returned to the High Court, this time to claim that he should be allowed to stay in Britain under the Human Rights Act. He again failed.

Last night Martin Bell, the former MP who won his seat on an anti-sleaze ticket, joined the attack. “Someone in Downing Street I think has had a common sense bypass” he said on BBC2’s Newsnight. “This really can’t go on. This is a Government . . . which came into power on the back of sleaze allegations. I think they have to get this right, and come out with the truth.”

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