Former Labour MPs have attacked their party's internal report on why it lost last year's General Election as a "litany of excuses" and "rubbish".

One ex-MP who lost his seat in Scotland in May accused the party of "failing to face up to where we went wrong".

The official inquest, led by Dame Margaret Beckett, was commissioned by the party leadership last year but has only now been made public.

It concluded that the party had the right policies, was not too left-wing, and that the 'SNP threat' had a decisive effect in a only a small number of seats.

It also found that 2014's Scottish independence referendum had left Labour’s hopes of outright victory “unachievable”.

Compared to 2010 Labour had neither “lost touch with middle class voters” nor its “traditional working class voters”, although it made little progress with either, it said.

However, the party had struggled to be trusted on the economy and to convince voters it had the answers on welfare and immigration

The report also warned that unless Labour recovers in Scotland by 2020 the party will need a swing from the Tories of 12.5 per cent to win.

One former Labour MP said: "This report unfortunately shows that some in the party are unable to face up to what went wrong - that we had a leader that many voters made their mind up about very early on.

And what they decided was that they could not see him as Prime Minister."

Another said: "This rubbish report, which is a litany of excuses, will not help us win in 2020."

The 'SNP threat' was a constant theme of Tory attack ads, including one which infamously depicted Alex Salmond as a pickpocket.

The report concluded that the attack may have reinforced the views some who had already decided not to vote Labour and "may have had a decisive impact in a small number of constituencies".

The report also says that Mr Miliband had been "clear" he would not do a deal with the SNP, despite complaints from Labour MPs before the final television debate that he had failed to out an informal coalition.

The report states: “Our view is that the seeds of defeat were sewn far before the short campaign."

It says the ambition to return to power after one term in opposition "was unachievable after the fall out of the Scottish referendum, regardless of leader and strategy".

The report claims that the rise in SNP support could not have been predicted because many of the party's policies “were more conservative than those of Labour.

"In fact, while some might have expected Labour to regain support after the referendum, the SNP adopted Labour policies they had previously rejected, and built not only on the emotion of the referendum, but on the perception of possible betrayal of government promises.”

The inquiry found that Labour lost because it won votes in the “wrong places”, mainly already safe Labour seats.

The party “may have been too ready to rely on the reassurance on the closeness of the polls, and either failed to hear the message from the doorstep, or, if it was heard, failed to act on it,” it warns.

The report rules out significant late switching – where voters change their minds between the last poll and the ballot box.

However, the party did find that large number of voters remained unsure of who would get their vote throughout the campaign.

An “unusual number" of them moved "in the same direction – away from Labour," it adds.

Meanwhile, it emerged that Labour is polling its worst post-election numbers since the Second World War.

Former minister Stephen Doughty, who dramatically announced that he had quit Jeremy Corbyn's shadow cabinet live on the BBC, said that the poll numbers were "deeply concerning to all those who want to see a Labour Government".