Education Secretary Michael Gove is to become the new chief whip in the most wide-ranging cabinet reshuffle of David Cameron's premiership.
He has been replaced by Treasury minister Nicky Morgan, as Mr Cameron promotes more women into top jobs.
Ken Clarke is among the old guard to have stood down and Foreign Secretary William Hague has moved to a lower profile role as Commons leader.
Defence Secretary Philip Hammond has taken over at the Foreign Office.
Among the other changes announced:
Lord Hill, Leader of the House of Lords, has been nominated as the UK's next European Commissioner
Liz Truss, a 38-year-old education minister, who entered the Commons in 2010, has been drafted into the cabinet as environment secretary
Business minister Michael Fallon - a veteran frontbencher - is named as the new defence secretary
Sir Bob Kerslake is to step down as head of the civil service in the autumn and will retire as permanent secretary at the communities department
Sir Jeremy Heywood, the cabinet secretary, is to take back the role of the head of the civil service - two years after the roles were split
David Jones is sacked as Welsh secretary to be replaced by his deputy Stephen Crabb
David Willetts replaced as universities minister by Greg Clark, who will attend cabinet
Justice Minister Jeremy Wright, a 41-year-old barrister, replaces Dominic Grieve as Attorney General, the government's chief legal officer
Baroness Tina Stowell, a former head of corporate affairs at the BBC, is the new leader of the House of Lords
Two MPs return to government: Mark Harper who quit as immigration minister after admitting employing an illegal immigrant as a cleaner, becomes a work and pensions minister, and Nick Gibb - sacked in a 2012 reshuffle - returns to the education department
The BBC News channel's chief political correspondent Norman Smith said the aim of the reshuffle was to bring in fresh faces and make voters think again about the Conservatives' "male, pale and stale" image.
The number of women in cabinet has gone up from three to five so far, out of a total of 22 ministers in Mr Cameron's top team. Ten women have been promoted so far, across government, with three taking ministerial jobs for the first time.
Former Defence Secretary Liam Fox has turned down David Cameron's offer of a return to government as a minister in the Foreign Office, saying on his website he preferred to carry on campaigning on immigration and Europe from the backbenches.
But it is Michael Gove's move to chief whip, a behind-the-scenes role in charge of party discipline, announced by Mr Cameron on his Twitter feed, that has caused the most surprise.
Mr Gove has been one of the most radical and at times controversial figures in David Cameron's government, driving through far-reaching changes to the education system such as free schools, exam changes and the extension of the academy programme.
His calls for a return to more traditional teaching methods and exam reforms have brought him into conflict with the unions - his replacement by the relatively unknown Nicky Morgan - seen as having a less confrontational style - may be an attempt to calm things down, said the BBC's Norman Smith.
Downing Street is talking up Mr Gove's new role, which will see him leading the fight against Labour in the general election campaign.
"You should expect to see a great deal of Michael Gove on your TV and radio channels," said the prime minister's official spokesman.
But BBC political editor Nick Robinson said it will be seen by some as a demotion as Mr Gove will no longer be a full cabinet member, just have the right to attend meetings when required.
"He now continues not as a leading Tory figure in his own right but as 'a friend of David and George'," writes Nick Robinson.
"His first job would appear to be to help them win the election. His second to make sure Boris and his old sparring partner Theresa May aren't the next Tory leader."
Nicky Morgan will add responsibility for equalities to her previous brief as minister for women, but business and education minister Nick Boles will be responsible for implementing same sex marriage legislation, which Mrs Morgan voted against on the basis of her Christianity.
The teaching unions have said they are seeking urgent meetings with Nicky Morgan to discuss pay, pensions and professional standards.
Ofsted chief Sir Michael Wilshaw said he was "surprised and shocked" by Mr Gove's departure, telling LBC radio: "I'm a great admirer of the Secretary of State, I think he's been a transformative and radical minister of education."http://www.bbc.co.uk/news/uk-politics-28302487
Working theory could be that Cameron is "disposing" of anyone with baggage before the sh*t really hits the fan.