ALISTAIR Darling's leadership of Better Together is reported to be coming under increasing pressure as opinion polls show a narrowing between the rival campaigns.

A senior Nationalist yesterday said the pro-independence campaign could draw level with the No camp in the opinion polls as early as next week, and not as previously predicted in July, because of gathering political momentum towards a Yes vote.

The SNP seized on more ­criticism of Mr Darling's leadership, with one senior Tory source saying: "He's a middlingly competent accountant with zero charisma. You never see him. Where is the big figure to lead the campaign and take the fight to Salmond? It's just dismal."

Conservatives are said to be arguing for Mr Darling to be replaced with Labour ex-Home Secretary Lord Reid, who, when he was touted as a potential No campaign leader in 2012, ruled himself out and has been barely seen in the campaign since.

The criticism of Mr Darling echoes similar comments made last year, which Prime Minister David Cameron tried to quash, insisting the former Chancellor was doing an "excellent job".

But last night Linda Fabiani, the SNP MSP, said: "With the No campaign's Tory paymasters starting to panic in the face of their falling poll ratings, Alistair Darling will no doubt be feeling that his position is under threat.

"But the No campaign's issues won't be solved by replacing a few personalities. Their fundamental problem is that they are stuck in a rut of their own negativity and bringing in members of the old guard, like Lord Reid, won't change that."

Over recent months several opinion polls have shown a narrowing between the campaigns. This has led some within SNP circles to suggest that two months out from the September poll the Yes and No campaigns will reach neck and neck. But now a senior Nationalist source has suggested the numbers will converge not in July but in May.

"The momentum is clearly with us. The UK Government and Better Campaign are floundering. Their negativity is catching up with them. We now expect the independence campaign to draw level within weeks," he said.

The best poll lead in February for the No campaign was 25 percentage points, the worst nine points. Last month, the best poll lead for the No camp was 15 points, the worst five points. This month, of six polls only two have had the anti-independence campaign with a double-digit lead. At the weekend, one poll had the Yes camp just three points behind.

SNP chiefs are hoping for a repeat of the swing before the 2011 Holyrood election. Labour had a healthy lead that was whittled away by the Nationalists to produce a landslide victory.

At Westminster, nerves are beginning to jangle within the Unionist camp, with some believing too much weight has been given to Tory Coalition figures and not enough to Labour Opposition ones; hence the arrival on Friday in Glasgow of Ed Miliband and his Shadow Cabinet, as well as the increasing presence of former Prime Minister Gordon Brown.

Labour sources have made clear the party and its leader have to "up their game" and maintain a healthy lead in the opinion polls for the 2015 UK General Election to ensure a referendum victory.