Boris Johnson today likened Nick Clegg to a worn-out "toy" and predicted the Liberal Democrat leader's bubble was about to "explode".
The London Mayor praised Tory leader David Cameron's performance in last night's leaders' television debate and said it was clear Prime Minister Gordon Brown was at the "end of his tether".
Speaking an election rally in West Hampstead, north London, Mr Johnson brushed off Mr Clegg's apparent rise in popularity.
He said: "The British people, they've been given a new shiny yellow kind of Fisher-Price Tonka toy.
"And they've bashed it around, they've been playing with it ... and after three weeks that Fisher-Price toy is starting to look a little bit tired.
"I don't wish in any way to be complacent but my distinct impression is the great Clegg bubble that has inflated over the last three weeks is about to explode."
Addressing party supporters while perched on a chair in a cafe, Mr Johnson said a vote for the Liberal Democrats would increase the likelihood of five more "miserable" years under Mr Brown by denying the Tories.
He added: "I think you saw last night that that's a man at the end of his tether. It would not be good for the country, it would be good for the Labour Party, it would not even be good for Gordon Brown. It would be inhumane."
To loud cheers from the small group of party activists, the Tory mayor saluted the way Mr Cameron handled yesterday's debate.
"Do you not agree with me that David Cameron walloped that one?" he said.
"I think the momentum is with us."
