Ed Miliband heads a new department for energy and climate change after the PM's reshuffle.
Mr Miliband will head the newly created department which will take over parts of the Departments for Environment, Food and Rural Affairs and Business, Enterprise and Regulatory Reform.
Mr Miliband has been Cabinet Office secretary since Gordon Brown became prime minister last year.
In another shock move, Peter Mandelson is returning to the government as Business Secretary, losing the energy portfolio to the new Department for Energy and Climate Change.
It has also been reported that chief whip Geoff Hoon is taking over the portfolio of secretary of state for transport vacated by Ruth Kelly.
The Guardian says Mr Brown could promote housing minister Caroline Flint to replace Hilary Benn in DEFRA - with deputy leadership candidate Jon Cruddas replacing her.