Gordon Brown is set to reveal who will be the new Health Secretary after Patricia Hewitt announced she was resigning for "personal reasons".
In a letter to the new Prime Minister, Ms Hewitt said she has decided to step down so she can give more time to her "constituency and family".
Although Mr Brown offered to keep her in the Cabinet, she insisted it was the "right moment" to go.
Ms Hewitt is a one-time left-winger who found no difficulty in fitting snugly into Tony Blair's New Labour project.
She was put in charge of the high-profile health brief following the 2005 general election, having been seen as relatively successful at the Department of Trade and Industry for four years.
However, despite massive government investment, her spell as Health Secretary was far from plain sailing
The 58-year-old was heavily criticised over her handling of NHS reforms and a financial crisis, which led to some trusts cutting back on staff and services.
In April last year, Ms Hewitt was jeered by nurses at a conference in Bournemouth, who were angry at her claims the NHS had enjoyed its "best year ever".
Alan Johnson, the current Education Secretary, has been widely tipped to take over the department.
The former postman, considered the most cheerful and unstuffy member of the Cabinet, has been a loyal Blairite throughout his relatively short political career after being made Work and Pensions Secretary in 2004.