Supporters of Conservative leadership challenger David Cameron say they have ruled out the possibility of a "dream ticket" deal with Ken Clarke.
A Cameron/Clarke alliance was touted earlier in the summer
Mr Cameron's team believe they can win enough votes without Mr Clarke and say the two differ over Europe.
Meanwhile, Mr Clarke told the journal Central Banking that the euro had failed to improve productivity and efficiency as much as he had hoped.
Ex-deputy PM Lord Heseltine is among the backers of the "dream ticket" idea.
'Most popular Tory'
Current Conservative leader Michael Howard said after the May election defeat that he would give up the role later this year.
Since then a series of potential candidates have been setting out their visions for the party, which has been in opposition since 1997.
Potential candidates also include Liam Fox, David Willetts, Theresa May, Sir Malcolm Rifkind, Andrew Lansley, Tim Yeo and David Davis, who is the bookmakers' favourite.
The rules will not be finalised for the leadership contest until late next month, although Conservative MPs have already voted for a system where they, rather than party members, get to choose the leader.
Lord Heseltine said Mr Clarke was "the most popular Conservative around".
"He is head and shoulders ahead of any other candidate. He's competent and the most popular and the most experienced," the peer told BBC Radio 4's Today programme.
While Mr Cameron was an "extremely personable guy", the Witney MP is also young and might welcome a period very close to the top "where he'd gain the experience of a professional bruiser which Ken is", he said.
"My real interest is helping the Tories to win. I think that Ken is capable of landing punches on this government in a way that I don't see anyone else being able to do."
Lord Heseltine said potential leadership contender David Davis was "enthusiastic" and "energetic" and would be "an effective leader".
"He's got a certain amount of experience as a junior minister in the last Conservative government, so I am not arguing against David Davis."
Tory MP Andrew Robathan, who backed Mr Clarke in the final round of the 1997 leadership contest, says he is now throwing his weight behind Mr Cameron.
"Time has moved on and I think it would be slightly surprising, to put it mildly, if the Conservative party were to choose one of Mrs Thatcher's Cabinet ministers to lead us into a general election probably in 2009/10," he told BBC Radio 4's World At One (Wato) programme.
While Mr Cameron was "inexperienced", he "shows remarkable ability, he's highly intelligent and he speaks well", Mr Robathan added.
Ed Vaizey, Tory MP for Wantage, said he did not expect Mr Clarke to win any leadership contest, despite the British public's "long-standing affection" for him.
He said it was time to choose a leader "who will represent the future", adding that Mr Cameron had been a "highly successful" MP who had served at "very high levels" in government.