A DAMAGING rift has split the campaign to keep Britain in the EU after senior Remain voices accused Nicola Sturgeon and Alex Salmond of harming their chances of victory in next month's poll.

Alan Johnson, the former Home Secretary and leader of the Labour In for Britain campaign, said SNP 'carping' was undermining the effort to secure a strong In vote.

READ MORE: Why the SNP's pro-EU allies are becoming increasingly frustrated by the party's friendly fire attacks

Willie Rennie, the Scottish Liberal Democrat leader, went further, accusing Mr Salmond of "risking Brexit" by criticising key parts of the economic case for staying in the EU.

He said the former first minister "should opt out of the campaign before he does any more damage".

Their comments were dismissed by a spokesman for Mr Salmond, who said the former first minister's "positive approach" would help the Remain side win the referendum.

Mr Johnson hit out following a series of friendly fire attacks by senior Nationalists on Chancellor George Osborne's official assessment the economic impact of leaving the EU.

In reports published this week, the Treasury warned Brexit could lead to the loss of more than 800,000 jobs and leave millions of present and future pensioners worse off.

READ MORE: Willie Rennie: Alex Salmond should quit EU Remain campaign

Ms Sturgeon said the claims were "overblown," while Mr Salmond, speaking during the first televised debate of the referendum campaign, repeatedly rejected the Chancellor's "apocalyptic" predictions.

Another senior Nationalist, MP George Kerevan, issued a scathing critique of the Treasury's case, saying it was "one sided and unfairly biased," and "riddled with approximations and inaccurate figures".

He said the UK Government's "Project Fear" approach played into the hands of the Leave campaign - but leading supporters of Brexit quoted his words extensively as they sought to rubbish the Chancellor's case.

Mr Johnson, who appeared alongside Mr Salmond in Thursday's BBC debate from Glasgow, said he backed the Treasury warnings because a string of independent experts and think tanks had reached similar conclusions.

Asked whether the SNP's criticisms were helpful to the Remain camp, he said: "I want to see Nicola Sturgeon and the SNP arguing vehemently for remaining in the European Union.

"There has been a lot of questioning figures, there has been a lot of 'this should be a much more visionary campaign' but not a lot of campaigning and setting out the arguments as to why it's crucial to remain in the European Union.

"That's what I want to see.

"I was happy to share a platform with Alex Salmond, this is above party politics.

"But we do need to be making the visionary case for Europe and carping about the figures just detracts from that."

Mr Rennie said: "Alex Salmond is behaving like a not-so-secret agent for the leave campaign.

"He spends more time rubbishing the official remain campaign and promoting Scottish independence than making the positive case to remain in Europe."

Responding to Mr Salmond's view that a second independence referendum would follow within two years of a vote for Brexit, he added: "His threats about another independence vote may encourage his supporters to back Brexit.

"His attacks on the remain campaign are undermining it and risking Brexit.

"On both counts he is risking our place in the European Union, not supporting it."

Ms Sturgeon has said Brexit could trigger a second independence referendum if the decision to leave the EU was not supported by a majority in Scotland.

However, she has been clear she wants the UK to stay in the EU and has vowed to fight a "positive" campaign for a Remain vote.

But Remain campaigners from other parties are becoming increasingly worried about the SNP's role in the debate.

One senior figure said: "Nicola Sturgeon and Alex Salmond have huge influence in this debate.

"Yet in every intervention they have made, they have criticised their own side.

"I'm sure they are committed to the EU but they cannot make a positive argument for a political union.

"The case is so similar to the case for Scotland staying in the UK, which they opposed two years ago.

"The language and rhetoric of the Leave campaign is so similar to the Yes campaign."

READ MORE: Iain Macwhirter: The sorry state of the Brexit campaign is not an excuse for ignoring the sorry state of the EU

A spokesman for Mr Salmond said: "By every account, Alex Salmond and Alan Johnson won the debate hands down for the Remain team.

"Their approach to the debate dovetailed perfectly, without indulging in any of the scaremongering which has bedevilled both Westminster-led campaigns to date.

"It is this positive approach which will win the referendum and perhaps explains why Remain is two-to-one ahead in Scotland but neck and neck across the country."

The spokesman said Mr Rennie was being "clownish".