Jack Straw has announced he is backing David Miliband's bid to become leader of the Labour Party.
The shadow justice secretary joins frontbenchers Alan Johnson and Alistair Darling in endorsing Mr Miliband for the leadership contest.
Mr Straw, who is stepping down from frontline politics, said the shadow foreign secretary had the "strength and depth" to guide Labour to power again.
Mr Miliband said he was honoured Mr Straw was supporting him.
In a letter to party members in his Blackburn constituency, Mr Straw wrote: "In my view there is one outstanding candidate with the qualities necessary both to be Leader of the Opposition, and then Prime Minister.
"In my judgment it is David Miliband who without doubt should be our next Leader. He will get my vote.
"He has the strength and the depth to stand up to David Cameron at Prime Minister's Questions week after week, and he can stand up for the people who will be most badly hit in our communities by the policies of this Con/Lib Dem Government."
Mr Miliband responded: "Jack Straw has worked with Labour leaders since Harold Wilson so I am honoured that he should judge that I have the qualities to return Labour to government."
Mr Straw previously nominated north London MP Diane Abbott for leader to ensure she made it into the final ballot.
Also standing in the leadership election are David Miliband's younger brother and former Energy Secretary Ed Miliband, former Health Secretary Andy Burnham and Ed Balls, who was schools and children secretary under Labour.
Labour MPs, Members of the European Parliament (MEPs), party members and members of affiliated organisations, such as trades unions and socialist societies, are all entitled to vote in the election.
Voting will take place between 16 August and 22 September, with the winner being announced on the first day of the party's conference in Manchester on 25 September.