David Cameron, Ed Miliband and Nick Clegg will miss Prime Minister's Questions to travel to Scotland to campaign for the union.

The unprecedented move follows indications in polls that Scottish voters may vote to leave the UK in the September 18 referendum.

In a joint statement from Mr Cameron, his deputy and the Labour leader, they say: "There is a lot that divides us - but there's one thing on which we agree passionately: the United Kingdom is better together.

"That's why all of us are agreed the right place for us to be tomorrow is in Scotland, not at Prime Minister's Questions in Westminster.

"We want to be listening and talking to voters about the huge choice they face. Our message to the Scottish people will be simple: 'We want you to stay.'"

The Scottish referendum outcome is "too close to call" as both sides enter the final days of the vote campaign, a research agency has said.

The TNS poll which puts the Yes and No campaigns equal on 41% came as the leaders of the three main pro-union parties put on a united front to pledge new powers for the Scottish parliament if it votes to stay in the UK on September 18.

The measures, which will be contained in a new "home rule bill", have yet to be detailed in full but the leaders of Scotland's Conservative, Labour and Lib Dem parties said they had agreed there would be more tax-raising powers for Holyrood.

It follows Gordon Brown's attempt to wrest back the initiative for the No campaign on Monday saying he was firing the "starting gun" for the introduction of far-reaching devolutionary changes.

The former prime minister said work on the transfer of power would begin on the day after the vote, should Scotland vote to remain in the union.

TNS said the Yes and No campaigns were neck and neck with 41% ahead of the vote on September 18.

Tom Costley, head of TNS Scotland, said: "This poll reveals a remarkable shift in voting intentions but the signs were evident in our last couple of polls which indicated a narrowing of the No lead, especially amongst those who told us that they were certain to vote.

"It is too close to call and both sides will now be energised to make the most of the last few days of the campaign and try and persuade the undecided voters of the merits of their respective campaigns."

Some 600,000 people could still have to make up their minds in the referendum, with leaders attempting to win over undecided voters.