Scottish Labour is very unlikely to win back the voters it lost to the SNP during the independence referendum in time for the general election, two new surveys suggest.

People who voted for independence have coalesced around the SNP and their vote is hardening, while Labour has become a party for No voters and its standing amongst nationalists has plummeted, analysis by the British Election Study (BES) has found.

Labour election co-ordinator Douglas Alexander said today that winning a majority will be "difficult but do-able", following a new YouGov poll for The Sunday Times which found that Labour has surged into a four-point lead across the UK.

But 1,300 Scottish voters polled by BES between March 6-13 puts Labour 17 points behind the SNP in Scotland, with the SNP on 44% and Labour on 27%.

Its findings are supported by a Scottish snapshot in the Sunday Times poll conducted on March 27-28, which puts the SNP on 46% and Labour on 33% amongst the 159 Scottish voters polled.

The BES report, by Jane Green and Chris Prosser, said: "It is Labour's hope that the losses it has seen to the SNP are temporary, and that those voters will come back to Labour in six weeks' time.

"Our data suggests that while not impossible, that prospect is very unlikely.

"Almost 90% of Scottish Labour voters are No voters and an equivalent 91% of SNP voters are Yes voters.

"This polarisation happened because of the movement of Yes voters away from Labour to the SNP and occurred between June and September 2014."

BES sees no way back for former Labour voters and also found the haemorrhage to the SNP is continuing apace.

"The proportion of Yes voters intending to vote SNP in September 2014 was 70%. That figure now stands at 79%.

"This trend is a worrying one for Labour in Scotland. Less than two months from the general election Yes voters are still moving to the SNP."

Labour defectors have also become more entrenched in their positions as SNP loyalists, BES found.

"This again is a worrying sign for Labour," it said.

"It suggests that Labour-SNP vote switchers are no more moveable than SNP loyalists. Among all UK parties, the SNP's vote base going into 2015 is the most certain on average of all.

"BES data cannot tell us what will happen in the coming six weeks. Vote switching happens beneath the surface of average percentages and the campaign could easily throw up unexpected results.

"But what we can say is that while Labour may convince its previous voters to come back to the fold, we do know that the odds are certainly stacked against them."

Speaking on the BBC's Andrew Marr Show, Mr Alexander said the SNP is trying "to drive up the Tory vote in England" to deliver an unpopular Conservative government that will boost support for Scottish independence.

"We have ruled out a coalition with the SNP and they have effectively done the same with Labour," he said.

"But as the polls indicate this morning, this is a winnable election for Labour.

"It is difficult, but it is certainly do-able that we can get a majority."

Lucy Powell, vice chairwoman of the Labour election campaign, told Sky News's Murnaghan programme: "The Tories have given up on a majority government after May 7 - we haven't.

"We still believe that there is every possibility that there could be a Labour majority government and that is what we are fighting for.

"We are doing incredibly well in many of the English and Welsh marginal seats, and the poll out today sort of confirms all of that, and we are sure that we can make sure that we win the seats in Scotland that we need to that we can win a Labour majority.

"The SNP obviously want to make themselves players in this election, but we are the only party that can beat David Cameron."

SNP Westminster leader Angus Robertson said it would be "presumptive" to envisage a scenario before the general election where the SNP could vote with the Conservatives to bring down a Labour government.

"First off, let's get ourselves into a position where we are able to hold the balance of power," he told the BBC's Sunday Politics Scotland.

"I have no plans to bring down any government.

"What I do want to do is have a government that is reflective of voters' wishes, and as we know voters in Scotland would prefer to see the SNP working together with the Labour Party if the majority allows that."

When pressed on whether the SNP would vote with the Tories to bring down a Labour government, he said: "I currently do not envisage any circumstances, and the reason why is that it is presumptive of politicians to even get beyond a scenario where we are trying to win an election."

But he also expressed concern at Labour's future spending plans.

"Douglas Alexander has today let slip where the Labour axe would fall - confirming his party are planning further cuts, including restrictions to the winter fuel allowance," he said in a statement.

"That is exactly why Scotland needs to vote SNP to achieve change which would benefit people across the UK.

"Labour recently voted with the Tories for £30 billion more austerity and they have made clear that left to their own devices, they would continue with cuts and waste £100 billion on renewing Trident nuclear weapons."