Ed Miliband will stand for the Labour leadership, the BBC has learned.
The former energy minister told members of his local constituency party in Doncaster that he intends to run and will announce it on Saturday morning.
His older brother David – the former foreign secretary – is also standing for the post vacated by Gordon Brown.
Ed said he had thought long and hard about standing against his older sibling, while David earlier insisted "brotherly love will survive".
The pair are the only two Labour MPs to have announced they are standing for the party leadership.
Speaking before Ed confirmed his decision David Miliband said he was "absolutely confident" the family could "remain strong" whatever happened.
Ex-Schools Secretary Ed Balls and backbencher Jon Cruddas have both indicated they are considering a bid.
Alan Johnson, Harriet Harman, Yvette Cooper and Jack Straw have ruled themselves out of the race.
David Miliband, a former protege of ex-Labour leader Tony Blair, has said he wants to rebuild the Labour Party as "the great reforming champion of social and economic change in this country".
He said Labour had to acknowledge that it had "lost this election and lost it badly" and now had to "re-engage with the public and understand, especially in England, why we lost".
When asked earlier in the week about speculation Ed would enter the race, he said: "We have talked very frankly and openly to each other because we love each other as brothers."
He added: "Brotherly love will survive because brotherly love is more important than politics."
Before he announced his intention to stand Ed’s supporters launched a website with a petition urging him to throw his hat into the ring.
The site says it has been set up by "a bunch of progressive activists who want to see a swift end to Tory rule".
Ed Miliband is less well-known than David and has been an MP for less time, but he has the backing of trade union Unite and is known to have supporters in the Parliamentary Labour Party.
David Miliband has already been backed by a number of Labour heavyweights including former Home Secretary Alan Johnson.
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