Defence Secretary John Reid says he has "no reason to believe" that there is a timetable for Gordon Brown to take over from Tony Blair as prime minister.
Mr Reid has worked with Mr Brown for 25 years
He sidestepped suggestions that the chancellor and Mr Blair have agreed the timings for a power handover, saying: "I have no knowledge of a deal."
The important thing was that Mr Brown was as committed to the New Labour project as Mr Blair, he told the BBC.
The Conservatives claim that Mr Brown has been a "roadblock to reform".
'Wait and see'
However, Mr Brown has signalled there could be more reform, not less, if he succeeds Mr Blair.
Former Cabinet minister David Blunkett said last month he believed there was a new "understanding" between Mr Blair and Mr Brown which will see the prime minister step aside within two years.
And there has been mounting newspaper speculation that the handover will take place within 18 months.
But Mr Reid told BBC Radio 4's Today programme: "I have no reason to believe that there is a timetable.
"I assume that we support the present leader of the Labour Party as long as Tony Blair is leader of the Labour Party.
"I have maintained the same position over these questions for the last five years - so if you will, permit me to wait until the prime minister goes."
He insisted he had no knowledge of any kind of deal between the two men. "If others feel that they have, they are free to say.
"The important thing is Gordon Brown is as committed to all of the New Labour principles as Tony Blair.
"That's important because that's precisely what the Conservatives and David Cameron try to suggest otherwise.
"I know because I have worked with Gordon Brown for 25 years and I know how committed he is to this project and to changing this country."