David Cameron signed a three party pledge that Scotland will quickly be given new powers if there is a no vote in the referendum, in a bid to spike Alex Salmond's guns before his TV debate against Alistair Darling today.

The joint statement offering extra tax and legal powers for Scotland has also been signed by Ed Miliband, the Labour leader, and deputy prime minister Nick Clegg, in an orchestrated effort to shore up the no vote.

Their signatures on the cross-party pledge – first endorsed by Scottish leaders of the pro-UK parties in June, will be unveiled in Glasgow on Tuesday morning as Darling and Salmond prepare for their first televised debate on STV.

The clash comes 44 days before the referendum on 18 September and see will both Darling and Salmond quizzing each other after making opening statements. The end of the debate will see both men being questioned by the audience.

The former Chancellor is expected to attack Salmond on doubts over North Sea oil, a Scottish currency and on public spending while the first minister will blame Darling for the recession, UK spending and welfare cuts, and his alliance with the Tories.

The debate will kickstart the final six weeks of campaigning to target the votes of the 500,000 or so voters who are crucial to the final result but have yet to decide, and are thought to be more sympathetic to greater devolution.

With the yes campaign trailing by about 12 points in the polls, Salmond is under the greatest pressure to win the contest; he is widely seen as the most accomplished and skilled debater of the two.

The pollsters Ipsos Mori found in June that Salmond has a popularity rating of plus 5 points while his deputy Nicola Sturgeon has a plus 15 rating, compared to a minus 16 rating for Darling, and a minus 36 rating for Cameron.

Published by Darling's Better Together pro-UK campaign, the joint statement commits Labour, the Tories and Lib Dems to give Scotland more power to raise its own taxes and to control parts of the social security system, within months of next May's general election.

Scottish Lib Dem leader Willie Rennie described the document as "a statement of common endeavour," but it was immediately attacked by the Scottish government.

"No one in Scotland will be fooled by this Westminster-led rehash of vague promises and unspecified more powers in the event of a no vote – the Tories have tried that before," Salmond's spokesman said.

Better Together seized on a new analysis of Scotland's likely finances in its first year of independence, mooted as 2016 by Salmond, from a recently launched thinktank Fiscal Affairs Scotland. The paper found that Scotland's deficit would be worse than the UK's by up to £900 per head unless oil revenues doubled over current forecasts or Scotland took only half its expected share of UK debts.

Jackie Baillie, Scottish Labour's shadow health secretary, said these findings added to the pressure on Salmond to defend his economic predictions for independence in tonight's debate. "Expert after expert lines up to explain the threat of separation to our public finances, Alex Salmond will have to explain why he is right, and they are all wrong," she said.