Former chancellor Kenneth Clarke has faced criticism from within the Conservative Party over his bid to succeed Michael Howard as party leader.
Ken Clarke lost to Iain Duncan Smith in the last leadership race
Ex-party chairman Lord Tebbit accused Mr Clarke of being "lazy" and warned voters would find his connections with the tobacco industry distasteful.
And rival Malcolm Rifkind highlighted his pro-Europe views as a weakness.
It comes as shadow foreign secretary Liam Fox suggested he would also be joining the leadership race.
In the most sharply worded attack, Lord Tebbit told the Mail on Sunday: "Ken may be the kind of bloke you would go out for a beer with but that does not qualify him to be a good leader."
"He will be smeared up hill and down dale as a merchant of death because of his long-standing connections with the tobacco industry. And he is just too lazy.
"He admitted he had not bothered to read the Maastricht Treaty when we were in power. That was typical."
He said Mr Clarke had "got it wrong" on Europe.
Sir Malcolm appeared to suggest Mr Clarke could divide the party if chosen as leader.
But he told Sky television: "It is going to be very important when the party reaches a final conclusion that whoever it chooses is someone who can lead a united party, who is acceptable even to those who didn't vote for that person."
And shadow chancellor George Osborne, who is backing shadow education secretary David Cameron for leader, hinted that the party needed more youthful and invigorating leadership than Mr Clarke could provide.
He told BBC News: "I think that Ken Clarke needs to make a powerful case to the party and I suspect he will do over the coming weeks.
"But the party needs to focus on who can lead us into the future, who is full of new ideas, who is able to connect us with younger voters."
A survey by the Observer newspaper of the 52 new Conservative MPs who entered parliament this year found none prepared to come out in support of Mr Clarke.
More than half said they had not yet decided who to back and only a few were considering the former chancellor.
Millionaire businessman Stuart Wheeler, a key party fundraiser, told the newspaper it would be dangerous if Mr Clarke did win the contest, saying: "He would split the party from top to bottom over Europe."
Dr Fox's aides say he will formally launch his leadership bid in a speech in London on Thursday.
He told the New of the World: "I am the only candidate who is going into this contest with a positive vision of the future."
He added: "I'm 43. I've been a minister and a Conservative Party chairman. I can provide a leadership that is relevant to all sections of the party and can reach out to our younger members and MPs."
Bookmaker William Hill made Dr Fox a 20/1 outsider.
They quoted Ken Clarke at 5/2 second favourite, behind the 4/7 favourite, shadow home secretary David Davis, who has yet to formally declare his intention to stand. David Cameron is 9/2 third favourite, with Sir Malcolm Rifkind at 16/1.
Hill's spokesman Graham Sharpe said: "Dr Fox's decision to run does not seem to have excited political punters."