The Campbells and the Blairs: A close-knit and formidable foursome at the top of New Labour

Alastair Campbell's departure from Downing Street is timed to coincide with that of his partner, Fiona Millar, and signals the break-up of the most important foursome in British politics.

While Mr Campbell battled at home and abroad at Mr Blair's side, Ms Millar offered invaluable advice as Cherie Blair's assistant and gatekeeper at Number 10. The "his-and-hers" combination worked well for years. The tightly knit unit formed a confidentiality at the top of New Labour that only close couples can achieve.

Both in opposition and government, Ms Millar was the perfect support to Mrs Blair as she attempted to balance her role as a mother, high-flying QC and party leader's wife.

Their two families looked extremely similar. Young, high-achieving, politically astute, each with three children (until Leo arrived) and living in cosmopolitan north London, the Blairs and Campbells had so much in common that it was no surprise that they gelled so well.

Ms Millar began working for Mrs Blair in 1995. Once Labour came to power, she acted part-time as an adviser until taking on the role full-time in 1998.

Ms Millar was formally "head of events" at Downing Street, but in effect she was in charge of Mrs Blair's public life. She would organise Mrs Blair's forays into charity work, speeches, teas with children at Number 10 and her consort's role alongside the Prime Minister on official trips abroad.

However, the couples have never been on holiday together and have not always seen eye-to-eye, the most notable incident in opposition being the Blairs' decision to send Euan to the selective Oratory school. Both Mr Campbell and Ms Millar are passionate believers in state education, with the latter a governor at two schools and all three of their children attending local schools. Earlier this year, Ms Millar went public with her worries on the school funding crisis that threatened to cut teaching posts at Gospel Oak primary school.

But the biggest crisis in the relationship came when Mrs Blair's "lifestyle guru", Carole Caplin, caused headaches for Downing Street through her boyfriend at the time, the convicted fraudster Peter Foster. When Mr Foster was reported to have helped the Blairs to buy two flats in Bristol, No 10 at first denied that he had advised Mrs Blair, until e-mails sent between the two revealed their association.

Before "Cheriegate" broke, Ms Millar and Ms Caplin are said to have fallen out over the hold the latter had over Cherie.

Mr Campbell also has little time for Ms Caplin, who is credited with teaching Mrs Blair "how to dress" when she moved into No 10.

But Ms Millar has told friends that Ms Caplin is not the reason she is leaving. She thought about resigning after the general election in 2001, but delayed after Anji Hunter's departure as Mr Blair's gatekeeper. Ms Millar has told colleagues that it is time to improve her "work-life" balance, insisting she doesn't want to stay in Number 10 "until I retire".

A former journalist who met Mr Campbell on the Mirror trainee scheme, she plans to resurrect her career in the media as well as work for educational foundations. Whatever she does, Downing Street's most effective mixed doubles partnership will be over for good.

A SHRINKING INNER CIRCLE

SIR DAVID MANNING

Mr Blair's chief foreign policy adviser has spent more time with Tony Blair than almost any other civil servant since 11 September 2001. Guiding the Prime Minister on the Afghanistan campaign and the Iraq conflict, Sir David's counsel has been "priceless", say insiders. His appearance before the Hutton inquiry was his last official act in his former role. He will soon depart to the US to become Britain's new Washington ambassador.

JONATHAN POWELL

Chief of staff to Mr Blair since 1994, the former diplomat has been a key part of the Prime Minister's support network. His formidable intellect and foreign experience, including his former posting in Washington, have also been invaluable as foreign policy dominated Labour's second term. Like Mr Campbell, he believes it is time to move on and has let it be known that he is in the market for job offers.

PETER MANDELSON

Mr Mandelson was one of the key architects of New Labour, known by ministers and opponents as the "Prince of Darkness". He transformed the party's presentation and strategy but was forced to resign from the Cabinet twice in separate scandals. Though he is unlikely to be granted another political comeback, Mr Mandelson is never far from the heart of power in Downing Street.