Mr Miliband had told the BBC last week that he was "not a candidate" to take over from Mr Blair, but political commentators said his remarks still left the cabinet minister room to later put himself forward in any ensuing leadership battle.
However in today's Observer Mr Miliband firmly rules out standing against Mr Brown when the prime minister leaves office at some point following the May 3rd local elections.
"I will vote for Gordon Brown to lead Labour's drive," writes Mr Miliband in a wide-ranging article about the party's future direction.
"I have watched him and worked with him for nearly 20 years. He has in the last ten years done great things for living standards; no-one is better qualified to lead across a wider canvas," the environment secretary explains.
Warning that achieving a fourth straight general election victory will be a "massive challenge" for his party, Mr Miliband stresses that the "real choice" is not between himself and Mr Brown, but between Labour and the Conservatives.
He warns that to win in a 2009/10 contest, his party will need to "resuscitate" the "sense of vision, hope and excitement" achieved by New Labour in 1997, but as part of a "broader and deeper" political project.
Mr Miliband adds that the government's record since coming to power needs "rehabilitation" and that Labour needs to be honest about its mistakes as well as the progress it has made in order to win over the public.
The environment secretary also warns that his party must "renew" its style as well as its policies and find new ways of connecting with voters if it is to achieve a fourth victory.
Meanwhile another Blairite minister who is seen as a potential leadership challenger appears to be keeping his options open in regard to standing against Mr Brown.
Home secretary John Reid told GMTV's Sunday Programme: "By saying that I won't discuss it, by definition you don't rule in or out."
"I'm sure the prime minister will make his views more specific, and when he does that, we will all have the chance to make sure that we express our views," he added.
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