THE scale of Labour's foreboding north of the Border has been laid bare on the eve of the first televised Scottish leaders' debate with one ex-MP and candidate admitting the SNP surge is "like a tsunami" even the world's best swimmer could not hope to survive.

As yet another opinion poll suggests a Nationalist landslide on May 7 with the SNP on course to achieve 47 seats, Labour candidates have been brutally candid about their chances of success.

One ex-MP said: "I'm now set to Defcon f****d. I'm expecting to leave and never come back. It doesn't matter how good you are or how weak your (SNP) opponent is; it's over."

It came as Alistair Carmichael, the Scottish Secretary, confirmed the leaked memo of a meeting between Nicola Sturgeon and French Ambassador Sylvie Bermann in February was written in the Scotland Office.

The Minister denied it was embarrassing, saying: "This is the middle of an election campaign; these things happen."

The memo claimed Ms Sturgeon had told Ms Bermann that she would "rather see David Cameron remain as PM'' and that she did not regard Ed Miliband as ''prime minister material''. The FM emphatically dismissed the claim as "100 per cent untrue" while Pierre-Alain Coffinier, France's consul general in Edinburgh, also denied it.

Sir Jeremy Heywood, the Cabinet Secretary, has now launched a leak inquiry although a Cabinet Office source could not say whether it would report before or after polling day.

The latest poll has Labour with just 11 seats out of a previous 40, the Liberal Democrats with just one of its previous 11 and Scotland again becoming a Tory-free zone in terms of the House of Commons.

Another former Labour MP admitted: "The polls are right. I hear it on the doorstep; my people hear it. We have thousands of conversations and the polls are bang on." He added: "I'll be looking for another career after May."

A third said: "It is like a tsunami; there's nothing you can do about it. It doesn't matter if you're the best swimmer in the world."

Tomorrow at 8pm, Jim Murphy, the Scottish Labour leader, faces arguably his toughest test yet when he goes head to head with First Minister Nicola Sturgeon - fresh from her success in the UK seven-cornered debate - as well as Ruth Davidson for the Scottish Conservatives and Willie Rennie for the Scottish Liberal Democrats in the first of three TV specials in Scotland.

The two-hour debate will be hosted by STV in Edinburgh's Assembly Rooms and chaired by the station's political editor Bernard Ponsonby.

It will be followed on Wednesday with a live one-hour BBC Scotland debate from Aberdeen again with the four party leaders together with Patrick Harvie for the Scottish Greens and David Coburn for Ukip.

As Ms Sturgeon again called on Ed Miliband to match the SNP's promise to "lock out" David Cameron from Downing Street even if the Conservatives won the most seats at the election, George Osborne raised the political stakes.

Describing the SNP leader's latest intervention as more evidence of an "unholy alliance" between Labour and the Nationalists, the Chancellor claimed a Lab-SNP pact after May 7 would mean Ms Sturgeon running the UK Government not Mr Miliband.

"People know (from seeing) Ed Miliband this week that the Scottish Nationalist leaders are much stronger than he is and we know who would be running our government.

"That would be bad for the entire UK, bad for the integrity of the Union but also bad for our economy because it would mean higher debts and higher taxes. Here in west London in Chiswick and Isleworth this is as much on the ballot paper as it is in Glasgow or Edinburgh," added Mr Osborne.

In response, Angus Robertson, the SNP's campaign chief, said: "Left to their own devices, Labour would simply pursue a further £30 billion of Tory austerity cuts but a strong team of SNP MPs will put backbone into Labour."

He added: "More anti-Tory MPs than Tory MPs at the election means that we can lock David Cameron out of Downing Street. And electing a strong team of SNP MPs will also ensure that Scotland has real power at Westminster to ensure that Scottish interests are served and progressive politics delivered across the UK."

In a separate development, a senior Ukip source said party leader Nigel Farage would not be coming to Scotland at any time during the election campaign despite the fact the anti-EU party is planning to field candidates in all 59 Scottish seats.