Alistair Darling’s poor performance in Monday’s TV debate has given a huge boost to Alex Salmond’s campaign for Scottish independence, a poll revealed last night.

The debate has been a disaster for the No campaign, which has seen its lead narrow sharply.

With only 20 days left until Scots vote, the pro-Union Better Together campaign led by Mr Darling still enjoys a six percentage point lead over the independence supporters among decided voters, according to the poll for the Scottish Daily Mail.

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The No campaign still has a six-point lead over the Yes campaign, but that is down from a 14-point lead enjoyed just three weeks ago after the first TV debate

Alex Salmond, the leader of the SNP, was widely judged to have triumphed during Monday night's televised debate against Alistair Darling

But this is down from the 14-point lead it had after the first debate three weeks ago.

If the shift in support towards the Nationalists continues at the same rate until polling day on September 18, it would see Scotland vote to leave the UK, ending the 300-year-old union.

However, the No campaign’s six-point lead now is the same gap as recorded in a poll carried out four weeks ago before the first TV debate.

As postal voting gets under way, growing numbers of Scots are making up their minds, with 53 per cent of decided voters set to vote No – down from 57 per cent in the last poll by Survation for the Mail on August 9.

The proportion of Scots planning to back separation has increased from 43 per cent to 47 per cent.

A large proportion of the voters were undeterred by the TV debate performance - but the impact looks like it will influence the final vote

When the 10.8 per cent of Don’t Knows are included, support for No was 47.6 per cent and for Yes 41.6 per cent, according to the poll of 1,001 adults aged 16 or above, which was conducted between Tuesday and yesterday.

In the previous poll, the figures were 50.3 per cent for No, 37.2 per cent for Yes and 12.5 per cent undecided.

Mr Salmond was the clear winner of Monday’s bad-tempered TV clash, according to the findings, with 64 per cent of respondents awarding him victory – including more than a third of No voters.

The First Minister will be delighted with the turnaround, but Unionist strategists can also take comfort because the BBC showdown has simply cancelled out the No campaign’s ‘bounce’ following the first head-to-head TV debate earlier this month.

After that debate was shown on Scottish television on August 5, which former Labour chancellor Mr Darling comfortably won, the campaign’s lead extended from six to 14 points among decided voters.

The referendum race is now almost neck-and-neck among men, but the No campaign maintains its commanding lead among women.

Among all age groups from 25 to 54, the Yes camp is in the lead, but this is countered by overwhelming support for the Union among older Scots, and strong support among youngsters.

Mr Salmond's dynamic performance, in which he faced down questions about currency while scoring points over Tory welfare cuts,has narrowed the No campaign's lead in the polls

Scots will vote in a referendum on September 18 to decide whether Scotland will remain part of the UK

The latest poll results came as Sir Mike Rake, president of the CBI, warned that Scottish independence could hinder the economic recovery across the whole of the UK.

Sir Mike, the chairman of BT Group and deputy chairman of Barclays, spoke about the ‘enormous’ risks of a Yes vote.

He told the CBI Scotland annual dinner in Glasgow that the CBI ‘does not see any substantive evidence for the position that independence would be economically beneficial for Scotland or the rest of the UK’.

He added: ‘Indeed, to the contrary, it is and will continue to create real uncertainty which could prejudice the recovery both in Scotland and across the United Kingdom.’

David Cameron, who was also at the dinner, warned that one million jobs rely on the Union and urged, ‘Let’s stay together.’

The Prime Minister said: ‘I want to talk about the business case for Scotland in the UK, something which matters to every man, woman and child in our country.

‘Our economy’s first great advantage is opportunity – the opportunity that comes from a domestic market of 60million people and nearly five million businesses.

‘This is one of the oldest and most successful single markets in the world.