Alistair Darling has warned the European Union referendum result is on a knife edge.

The former Better Together boss said that the vote's outcome was "too close to call".

His warning came as the Remain and Leave camps both accused each other of putting the NHS at risk.

On the first official day of campaigning, a leading pro-Brexit Tory MP also claimed that Barack Obama, who is expected to make a plea for the UK to stay in the EU, had appointed IRA sympathisers to his top team.

Today the Scottish Vote Leave camp will also claim that MSPs are being “gagged” over the EU.

During a speech in London, Mr Darling said that “every single vote counts” in June's referendum.

His comments will be seen in part as a rallying call to Labour supporters.

There have been fears that labour leader Jeremy Corbyn’s apparently lukewarm support for staying in the EU could encourage some voters to sit at home.

Mr Darling also hit out at those who back leaving the EU, which he predicted would be a "self-inflicted wound".

He said: “This is a very, very close vote. I really can’t emphasise enough that the vote is close

“No one can predict with any certainty what is likely to happen.

“I hope we will win and I hope we will win well. But we need to get the support of people the length and breadth of the country, no matter what their political allegiance has been in the past.”

Mr Darling also rejected the 'Project Fear' tag, accusing his opponents of 'Project Fantasy', as he suggested that his opponents were putting the future funding of the NHS at risk by putting the economy at risk.

Earlier the pro-Brexit camp had claimed that the money it says the UK will save by leaving the EU to go into the health service.

Prominent Leave campaigner and Tory MP Jacob Rees-Mogg also branded senior Obama administration figures "hostile to the UK" saying some had held up an extradition treaty in the senate in the 1980s "because they were sympathetic to the IRA."

Mr Obama is expected to make a major intervention in the campaign, backing David Cameron's bid to remain in the EU.

Boris Johnson also hit out, saying it was "hypocritical" for America to encourage the UK to stay in the EU.

Mr Harris, the director of Scottish Vote Leave, will today claim MSPs are effectively being gagged by pro-EU party leaders.

Mr Harris said that he had been in contact with a number of SNP, Labour, and Tory parliamentary candidates “who are eager to campaign for a Leave vote, but feel unable to make their true feelings known before May".

He added that the situation was a "betrayal of the Scottish people".

The 10-week official campaign will place strict limits on how much cash each side can spend trying to persuade voters to back their cause.

There was controversy earlier this month, however, when Mr Cameron spent £9m sending a pro-EU leaflet to every house in the country, just before the rules kicked in.