David Miliband would become a "human sacrifice" if he stood against Gordon Brown as Labour leader, Foreign Secretary Margaret Beckett has warned.
Mr Miliband has said Mr Brown is an excellent PM-in-waiting
The environment secretary would be "daft" to take on the chancellor "because of someone else's prejudices", Mrs Beckett told the Daily Telegraph.
She said Mr Brown was "head and shoulders" above any other candidate.
Mr Miliband, a strong supporter of Tony Blair, has repeatedly said he believes Mr Brown is an excellent PM-in-waiting.
In an interview, Mrs Beckett said the 41-year-old would be unwise to put his career at risk by running against Mr Brown, who is seen as the strong favourite to win.
She added: "If Gordon fell under a bus tomorrow we have got a lot of potential leaders, but it is no discredit to any of them to say that Gordon is head and shoulders ahead of those, so what is the point of having an artificial contest for the sake of it?"
So far only two MPs - left-wingers Michael Meacher and John McDonnell - have declared they will run for the Labour leadership.
Earlier this month, former Home Secretary Charles Clarke urged Mr Miliband to stand, saying he would be a "good candidate and a good prime minister".
He said this was not about "stopping Gordon Brown" but "promoting serious policy and political discussion".
But Ms Beckett said Mr Miliband would be ill-advised to run, adding: "I think, 'Poor David'. It is complete nonsense.
"He is very bright. He is a very good minister.
"I am sure that he will be a major contender at some time in the future.
"But should he take this step to satisfy someone else's prejudices? He would be daft."
Ms Beckett was herself beaten by Mr Blair in the contest to become Labour leader in 1994.