Hilary Armstrong has become the latest minister to announce she will step down from office once Tony Blair resigns.
The former chief whip, now Cabinet Office minister, said she wanted to give the next prime minister Gordon Brown "space in his Cabinet to renew".
But she said Mr Brown, who will take over on 27 June, could rely on her future loyalty from the backbenches.
Deputy prime minister John Prescott and Home Secretary John Reid are also stepping down when Mr Blair goes.
Ms Armstrong, 61, a close ally and neighbouring MP of Mr Blair, said: "It is important for any government to renew after 10 years in power, whoever is prime minister.
"I want to give Gordon Brown some space in his Cabinet to renew"
"As a former chief whip, I know the difficulties a prime minister faces at a reshuffle with limited space for manoeuvre.
"I want to give Gordon Brown some space in his Cabinet to renew.
"I have had the privilege of working with Neil Kinnock, John Smith and Tony Blair. They always knew they could completely depend upon my loyalty. It will be the same for Gordon Brown."
The Durham North West MP was minister for local government for four years before being made chief whip in 2001.
She came under fire in February 2006, when Mr Blair missed a crucial vote on plans to introduce a new offence of inciting religious hatred - which the government went on to lose, by one vote.
She was later moved to become social exclusion minister, but kept her Cabinet seat as chancellor of the Duchy of Lancaster - the minister in charge of the Cabinet Office.
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