Gordon Brown will show "political cowardice" if he does not call a snap general election, William Hague says.
The ex-Tory leader told BBC Radio 4's Today programme that a delay would mean Labour feared it could not win even during Mr Brown's "honeymoon period".
The prime minister has refused to rule out an election this autumn but says he already has a mandate to govern.
Labour, the Conservatives and the Liberal Democrats all say they are ready to mount a campaign.
According to reports, Mr Brown is planning to bring the Comprehensive Spending Review - which sets government spending budgets for the next three years - forward to early next week.
In theory he could call an election on Tuesday next week to be held on 1 November, having also made an announcement about troop levels in Iraq.
Mr Brown has until 2010 to call an election, but positive showings in recent opinion polls have prompted speculation he might do so this autumn.
Amid the mounting speculation he has refused to rule out holding an early election, saying he was concentrating on the job of prime minister he has been doing since Tony Blair stepped down in June.
Shadow foreign secretary Mr Hague said: "Clearly if he [Mr Brown] were to step back from having an election now, then dithering would have turned a degree of political cowardice on top because the expectation had been raised so high.
"And I think it would be a concession since the Labour Party would have an election now if they thought they could win it, that's pretty clear.
"It would be a concession that they don't think even they can win an election now with Gordon Brown's remaining honeymoon period."
Last week, Labour treasurer Jack Dromey said the party was "gearing up" for an election, declaring: "I'm confident of our capacity to be ready. We will be."
But Home Secretary Jacqui Smith said Mr Brown's focus was "getting on" with the job of being prime minister.
On Monday, Sir Malcolm Rifkind, a former Conservative foreign secretary, said: "I think to contemplate a general election two years after the last general election when you have a healthy working majority in the House of Commons is a constitutional outrage."