Last night Better Together leader Alistair Darling said he was "absolutely confident" Scots would vote to stay in the UK, but he conceded the next two weeks would be a "race to the wire".
His comments came as Labour and the Liberal Democrats prepare to step up the activity of a number of "big beasts", including former Liberal leader Charles Kennedy, as well as former Prime Minister Gordon Brown and former Defence, Health and Home Secretary John Reid, in an attempt to win back disaffected Labour voters thinking of voting Yes.
Mr Kennedy will join Mr Darling today to campaign on doorsteps in Inverness.
Yesterday Sir Edward Leigh, Tory MP for Gainsborough, told Mr Cameron the campaign up until now had been "complacent".
The Conservative leader is expected to campaign in Scotland before voters go to the polls on September 18. But Downing Street sources have already indicated the Prime Minister will not be north of the Border on polling day, saying he would leave the final push to the different anti-independence campaigns.
During Prime Minister's Questions (PMQs) in the Commons, Sir Edward told Mr Cameron: "For all the reasons that have been given, if we were to lose the Union, it would be not only a disaster for Scotland, but a national humiliation of catastrophic proportions."
He then went on to criticise the campaign in an attack directed at the Labour leader Ed Miliband and the Liberal Democrats' leader Nick Clegg, as well as Mr Cameron.
He said: "I say gently to the three party leaders that perhaps we have been a bit complacent up to now.
"I urge them, over the next two weeks, to drop everything else and stand shoulder to shoulder to fight for the Union we love and believe in."
In response, Mr Cameron denied the charge, saying the three leaders had all put aside their differences to campaign against independence.
The Tory leader also used PMQs to accuse First Minister Alex Salmond of telling "lies" that Westminster poses a threat to the future of the devolved NHS in Scotland.
Mr Cameron also denounced Mr Salmond's pledge to walk away from any share of national debt unless the remaining UK agreed to a currency union as "chilling".
"We all know what happens if you do not pay your debts - no-one will lend you any money unless you pay a punitive interest rate," he said.
"We all know what that means for homeowners - much, much higher mortgage rates. For businesses, it means crippling interest rates.
"Those are the consequences of what the separatists are proposing. We need to get that message out loud and clear in the coming days."
Sources close to Mr Salmond said the high profile given to the Scottish independence debate during the weekly joust in the Commons showed how "rattled" pro-Union politicians are, with recent polls suggesting only a small gap between Yes and No.
Mr Cameron also said he had had talks with STV over a television programme with undecided voters.
The Prime Minister suggested the broadcaster "ran away" from the idea during a discussion over dates and formats.
But a spokeswoman for STV said the idea was still on the table. She said: "We are aware undecided voters would appreciate the opportunity to put their questions to the Prime Minister before the referendum vote and, as such, STV's programme offer still stands."