Miliband early front-runner in race to replace Brown

Mr Miliband became the first Labour MP to confirm his candidacy for what will be the party's first leadership election in 16 years after receiving the heavyweight backing of former Home Secretary Alan Johnson, previously one of the favourites to land the role.

The jostling for positions within the Labour Party which has been rumbling behind the scenes for at least two years moved into overdrive after Mr Brown's dramatic resignation on Tuesday night. While no other candidates have so far formally declared, Mr Miliband could well face an intriguing battle with his younger brother Ed, the MP for Doncaster North.

Morley and Outwood MP Ed Balls is also widely expected to put his name forward, though sources close to the former Schools Secretary said he would not be entering the race yet. A challenge from the left of the party is also anticipated, with backbenchers Jon Cruddas and John McDonnell considering possible bids.

A timetable to find Mr Brown's successor will be put in place over the coming days, and acting leader Harriett Harman – who has ruled herself out of the leadership contest – told MPs: "With great dignity Gordon Brown resigned the leadership of the Labour Party, but his legacy – our legacy – will endure. We will be a responsible opposition and we will be an effective opposition ...We will be a powerful opposition."

Launching his leadership bid, David Miliband said that in the face of coalition government Labour must be the "great unifying force of all shades of centre and centre-left opinion".

His bid received a huge boost with the backing of Mr Johnson, who called for "the broadest possible contest" in contrast to Mr Brown's unchallenged succession in 2007. "I think we have a bevy of great talent there," he said. "I just think David is the greatest talent."

Mr Miliband also received the backing of Don Valley MP Caroline Flint, who said a new leader must be in place "by the summer" .

Penistone and Stocksbridge MP Angela Smith also backed Mr Miliband, while Central York's Hugh Bayley and Wakefield's Mary Creagh attended his launch event last night.Other Labour MPs who have ruled themselves out of the contest include Mr Balls's wife Yvette Cooper, the MP for Pontefract and Castleford, former Wales Secretary Peter Hain and former Communities Secretary John Denham.