Ex-Home Secretary Charles Clarke will not run for Labour's top job when Tony Blair steps down, he has told the Times newspaper.
Charles Clarke had set up a website to encourage policy debate
Mr Clarke said he would have had the support to be nominated as leader, but had decided there was "no appetite for that kind of contest in the party".
With Gordon Brown expected to take over as PM, Mr Clarke said he would like to serve as a member of his Cabinet.
Any challenge to Mr Brown is likely to be from one of two left-wing MPs.
Mr Clarke told the Times: "I am not going to run in the leadership election. I was ready to run. I was considering running. I think I would have had the support to be nominated and be able to run."
He added: "There is not the appetite for that kind of contest in the party at the moment."
Asked whether he would serve in a Brown Cabinet he replied: "Yes is the answer to that question. I've enjoyed being in government... if it would be useful for me to serve I would like to do that."
In February, Mr Clarke and former Health Secretary Alan Milburn launched a website, The 2020 Vision, to debate future policies with both the party and the public.
At the time, Mr Clarke said it was not about "stopping Gordon Brown" but "promoting serious policy and political discussion".
Mr Clarke's announcement comes after Culture Secretary Tessa Jowell predicted no senior minister would challenge the chancellor either.
Labour MPs Michael Meacher and John McDonnell - who had been planning separate leadership challenges - last week announced they were uniting in their bid to target Mr Brown.
They agreed one of them would withdraw, depending on the level of backing they had when Mr Blair steps down.
Mr Blair said earlier this week that "in all probability" Mr Brown would succeed him as prime minister "in the next few weeks".
If there is only a challenge from a left-wing candidate, Mr Blair is expected to come out in support of Mr Brown once the contest to replace him gets under way - although he has yet to formally back him.
Mr Blair is to announce his resignation next week, after which he will stay as PM for about seven weeks until Labour chooses his successor.