Key relationship at the heart of Downing Street damaged after Campbell's advice was ignored

They are the Blairs' most loyal advisers, rallying to their defence at the slightest hint of political trouble or whiff of media criticism. But there are signs that Alastair Campbell and his partner, Fiona Millar, have been angered by the behaviour of the Prime Minister and his wife over the purchase of the Bristol flats.

Mr Campbell and the Labour leader go back a long way. Since 1994, Mr Campbell, a former Mirror political editor, has worked hard to protect Mr Blair from the sort of media attacks that all but destroyed Neil Kinnock. He has been unfailingly at Blair's side and available for the most confidential political advice, earning the tag of the "real prime minister".

His long-term partner, Ms Millar, director of events and visits at No 10, and Mrs Blair's media adviser, has relentlessly steered away harmful press scrutiny from the Prime Minister's wife, protecting her from scornful tabloid attacks.

But last week, Downing Street's sentinels were caught off guard by the bizarre story of Cherie Blair's friendship with a former soft-porn model – Carole Caplin – and her business dealings with her convicted fraudster boyfriend, Peter Foster. When a tabloid newspaper produced proof that the Prime Minister's wife had been busily e-mailing Mr Foster, who was also facing deportation, Mr Campbell and Ms Millar were incredulous. Blair aides say the fact that Mr Campbell was not told about the Foster connection was a rare breach of trust.

As the first editions hit the streets on Wednesday night, Mr Campbell and his wife were involved in an 11th-hour attempt to unravel the truth and protect the Blairs' reputation from further damage. They even insisted the Prime Minister's wife open up her personal e-mail account late at night so they could read the damaging e-mails.

In a rare critical outburst, Mr Campbell is reported to have told friends: "You can't have a convicted fraudster anywhere near the Prime Minister's inner circle." He and his partner are said to be furious they were not kept in the loop about Mrs Blair's dealings with Mr Foster and were forced on the defensive when newspapers produced evidence that Mrs Blair had not only asked his advice but had called him "a star".

What was even more annoying was that Mr Campbell had warned Mrs Blair months ago that her friendship with the so-called lifestyle guru, Ms Caplin, could be compromising to the Prime Minister.

Mrs Blair is reported to have resented Mr Campbell's intervention into her private life, insisting that her friendship with Ms Caplin was her own business, and considering his inquiries little better than tabloid intrusion. "She treats us as though we are in the same category as the Daily Mail," Mr Campbell told a colleague. Ms Millar is said to be so disgruntled that she is reported by friends to be "reassessing her position" at Downing Street.

For the director of communications a quandary presented itself: how could he protect the Prime Minister when the source of damage was his wife? His usual advice would be to distance himself or sack the culprit but that was clearly not possible.

Last week's events brought into the open the first signs of serious tension between the Blairs and their closest protectors. Although Mr Campbell is unlikely to quit over the incident, when he does eventually step aside, last week is certain to be identified as a fault line in the relationship.