THE ­pro-independence campaign has predicted the ­referendum will go down to the wire, with support for a Yes vote trailing the No side until just days before the ballot on September 18.

In a marked retreat from recent predictions that it could go into the lead within weeks, a new polling analysis from Yes Scotland said it would not be ahead until September.

Blair Jenkins, Yes Scotland's chief executive, last night said he remained confident of crossing the winning line, but added: "We do not underestimate the challenge ahead."

Jenkins was speaking after ­issuing an analysis of polls from the last six months that showed a steady increase in support for a Yes vote and a month-by-month decline for a No.

Leaving out undecided voters, support for a Yes vote has risen from an average of 38% in November last year to 45% by April.

Meanwhile, the average support for No has fallen from 62% in November to 55% in April, meaning the gap between the two sides has narrowed from 24 percentage points to 10.

The analysis, which covers 26 polls up to and including this week's YouGov poll, which showed the No side appearing to recover ground, stated: "The trend is steadily moving towards a Yes. When this is extrapolated forward it shows Yes taking the lead by September."

Jenkins said: "We know that we still have a lot of hard work to do between now and September 18 to attract more people to Yes, but we are clearly making steady progress and I am confident we can win. This is because as more and more people becoming engaged in the debate, they see that the case for taking charge of our own future is not only something we can do, but should do."

A similar analysis by the ­Financial Times, which tracked polls since the start of the year, showed a slightly narrower gap, with Yes on 46% and No on 54% once Don't Knows were excluded.

Jenkins added: "Of course, there is still a gap that we need to close and we do not underestimate the challenge ahead.

"We are very much up for that challenge and will be doubling our efforts."

The SNP yesterday seized on reports that David Cameron was to make a series of visits to Scotland to promote the Union, claiming it was evidence the Labour chairman of Better Together, Alistair Darling, was being sidelined.

The increased role for the Prime Minister follows anonymous sniping by senior Tory figures that Darling is a dismal and dull campaigner.

SNP MSP Linda Fabiani said the Better Together parties were showing signs of panic. "The No campaign is clearly in deep trouble if their response to their falling poll ratings is to draft in the unpopular Tory leader. This move is a clear indication the No campaign's Tory paymasters have lost all confidence in Alistair Darling after criticising his 'dismal' leadership - it is even clearer now that the No campaign is simply a Tory front.

"If David Cameron wants to play a real part in the debate on Scotland's future he should have the courage of his convictions and finally agree to a debate with the First Minister."

Reacting to the polling analysis, Better Together said: "Every poll shows that the majority of people in Scotland want to have the best of both worlds: a strong Scottish Parliament with more powers guaranteed and the strength security and stability that come from being a part of the bigger UK."