Gordon Brown will end up looking like "a big girl's blouse" if he does not call an autumn general election, Boris Johnson told the Tory conference.
Mr Johnson did not say if he would defend his Henley seat
The newly adopted Tory candidate for Mayor of London urged the prime minister not to "wimp out".
But he refused to say whether he would defend his Henley seat or stand down to concentrate on the mayoral contest.
Mr Brown has refused to rule a snap election in or out, but has insisted he is just "getting on with the job".
Speculation has been mounting that an election announcement is imminent - as Mr Brown is thought to be considering capitalising on Labour's lead in the opinion polls.
Last week it emerged that Labour had started recruiting people who would play an important role in a general election campaign.
An early election would pose a dilemma for Mr Johnson - if he resigns as an MP to concentrate on taking on London's mayor Ken Livingstone, he risks having to leave frontline politics if he fails to become mayor.
Asked whether he would stand in Henley again, if there is an autumn poll, he said: "This question is better directed at the great quivering jelly of indecision that is Gordon Brown.
"He's sat so long on the fence that the iron has entered his soul."
But he told a fringe meeting at the Conservative Party conference in Blackpool that Mr Brown had to make a decision.
"If he fails to call it now then what will we say? We will say he's wimped out, we will say he's a big girl's blouse."
Last week shadow chancellor George Osborne said Mr Brown will have "bottled it" if he does not make an announcement imminently.
But others in the Conservative Party believe Mr Brown should not call an early election - earlier former foreign secretary Sir Malcolm Rifkind said governments with a working majority in the Commons should serve a full term.
Sir Malcolm said calling an early election in such circumstances would be a "constitutional outrage".