Harriet Harman has flatly denied plotting to succeed Gordon Brown - and ruled out entering a leadership contest in the future.
The deputy Labour leader said she would not seek the top job even if the Prime Minister were to step aside.
The intervention came after a week of bitter infighting that has left Mr Brown's position looking increasingly precarious.
He suffered his first Commons defeat since entering Number 10 in an emotive vote on Gurkha rights and was forced to stage a double climbdown on reform of MPs' expenses.
The problems were exacerbated on Sunday when Communities Secretary Hazel Blears mocked his much-derided YouTube video in a newspaper article and Health Secretary Alan Johnson indicated there were circumstances where he might seek the leadership.
Ms Harman went on a tour of TV and radio studios following reports that she was preparing to take over if a vacancy arose.
The Cabinet minister told BBC Radio 4's Today programme: "I am saying there are no circumstances. I do not want to be Prime Minister, I do not want to be leader of the party.
"I want Gordon Brown to remain PM after the next election as well as before the election."
Ms Harman said she did not believe Mr Brown would be forced from the leadership.
"There is not going to be a leadership election. There should not be, and there will not be," she added.
