Former Health Secretary Alan Milburn is back in the Cabinet, Downing Street confirms as Tony Blair's reshuffle gets under way.
Milburn quit the Cabinet last year
Arch-Blair ally Mr Milburn will take a lead role in Labour's election campaign and take the official title of Chancellor of the Duchy of Lancaster.
Higher Education Minister Alan Johnson has also been named as the new Work and Pensions Secretary.
No 10 says changes among junior ministers will be unveiled on Thursday.
But officials say there will be no more changes at Cabinet rank and they confirmed
Labour Chairman Ian McCartney had kept his job.
Wednesday evening also saw former Cabinet minister John Redwood return to the Tory front bench as Michael Howard conducted his own reshuffle.
Mr Milburn left Downing Street at around 2100 BST smiling broadly and ignored reporters' questions.
He had earlier entered No 10 through the Cabinet Office and spent much of the evening locked in talks.
Johnson is the first ex-union leader in Cabinet for 40 years
BBC political editor Andrew Marr said Health Secretary John Reid and Culture Secretary Tessa Jowell had earlier urged Mr Milburn to accept the job.
His appointment is already being seen as a victory for Mr Blair over Chancellor Mr Brown, who was widely thought to be opposed to Mr Milburn taking a big election role.
Election planning has traditionally been Mr Brown's domain but the chancellor's allies insist he is relaxed about the appointment.
Mr Milburn will be based in the Cabinet Office and Downing Street sources he will supervise
the work of the No 10 policy directorate and the prime minister's strategy unit, as well as "co-ordinate the development of policy across government".
Douglas Alexander previously held the post of chancellor of the Duchy of Lancaster outside the Cabinet.
No 10 says details of how the move might affect Mr Alexander, who is also a Cabinet Office minister, will be released on Thursday.
Mr Johnson had also been widely tipped for his promotion after the surprise resignation of Andrew Smith as work and pensions secretary on Monday.
He is the first former union leader in 40 years to become a Cabinet minister.
But he is also known as a loyal Blairite who earned the prime minister's respect for the way he steered through Labour's plans for university top-up fees in the teeth of disquiet on the back benches.
The Tories contrasted their surprise changes with the "endless bickering" over Tony Blair's impending reshuffle
Former Welfare Minister Frank Field said Mr Milburn's appointment had been inevitably after the days of speculation.
"It would have been a total defeat for the prime minister had he not returned given that No 10 had been briefing all week that he was going to return," Mr Field told BBC News 24.
The MP said Mr Johnson was the right man for the pensions role but Mr Blair would still need to persuade his chancellor to accept long-term reform of pensions policy.
Tory chairman Liam Fox contrasted the Tory changes with the delays over Labour's reshuffle.
Dr Fox added: "By giving Alan Milburn a non-job, Tony
Blair has wimped out of a confrontation with his own Chancellor. What a pathetic
and humiliating position for any PM to find himself in."
Downing Street has stressed Mr McCartney has Mr Blair's full backing as Labour chairman and Cabinet minister.
There had been newspaper reports that Mr McCartney was to be dropped but it is thought Deputy Prime Minister John Prescott lobbied strongly on his behalf.
Mr McCartney used an interview with the Guardian newspaper to attack "ill-discipline and briefing within the Westminster village".
He added: "The most difficult part, and maybe the part which was unacceptable, was when people brief against you on a personal basis."