Corbyn's aides have long insisted that their man can win a general election.

Now, thanks to Theresa May, they will have the chance to find out whether or not that is true.

The omens do not look good.

Opinion polls suggest that Labour is trailing the Conservatives by more than 20 points.

The first poll conducted since the snap election was called brought more bad tidings, giving the Conservatives another 21-point lead.

The ICM survey put the Conservatives on 46 per cent, Labour on 25 per cent, the Liberal Democrats on 11 per cent, Ukip on 8 per cent and the Greens and SNP both on 4 per cent.

ICM’s Martin Boon said that the result showed that “Labour have an electoral mountain to climb, and its leadership appears to have left its ropes and crampons at base camp”.

Across much of England the party will have to try to hold seats where Labour voters backed Brexit in huge numbers, despite the party's call for a remain vote.

In Scotland the party also faces a difficult challenge.

At the last general election, Ian Murray, Labour's sole MP north of the Border, increased his majority in Edinburgh South from 316 to 2,637.

But he will be a high-profile target for political opponents keen to say that Labour has suffered a 'wipeout' in Scotland.

Labour was already braced for a difficult electoral challenge in the next few weeks.

Across the UK Labour is predicted to lose up to 125 councillors at the local elections next month.

The figure is extraordinary in part because opposition parties usually increase the number of councillors they have.

Corbyn supporters point to the fact that the polls were famously wrong in the run up to the 2015 general election.

But polling experts warn that that was because they generally underestimated Conservative, not Labour, support.

However, there are suggestions that voters have responded well to some of the party's recent policy announcements.

And one opinion poll at the weekend, by Opinium, gave the Tories a much narrower lead of nine points.

Nevertheless as Labour MPs vote today to hold a general election in June, many of them will do so in the belief that they will be putting either themselves or fellow Labour MPs out of a job.