William Hague has stood down as foreign secretary and will stay in the cabinet as Leader of the Commons, Downing Street has said.
Mr Hague is to leave Parliament at the 2015 general election after 26 years as MP for Richmond, North Yorkshire.
As part of a significant ministerial reshuffle, veteran Conservative MP Ken Clarke has also stepped down.
Senior ministers have told the BBC that the current defence secretary, Philip Hammond, will replace Mr Hague.
Labour described the reshuffle as "the massacre of the moderates".
Prime Minister David Cameron said: "William Hague has been one of the leading lights of the Conservative Party for a generation, leading the party and serving in two cabinets.
"Not only has he been a first class foreign secretary - he has also been a close confidante, a wise counsellor and a great friend.
"He will remain as First Secretary of State and my de facto political deputy in the run up to the election - and it is great to know that he will be a core part of the team working to ensure an outright Conservative victory."