Shadow Cabinet Minister Jon Ashworth has called projections of a 12-point swing from Labour to Conservative by an elections expert "pretty depressing" and has called on the party to step up efforts to win key marginal seats in the upcoming local elections.

Professor John Curtice delivered his analysis after noting Labour's position in the opinion polls weakening over the last year.

The elections will be held on 4 May and Ashworth outlined a handful of significant regions that must be won in order to cut the gap on the Tories.

"We have to be winning seats. We cannot be falling back on the scales that have just been suggested," he said on BBC One's Sunday Politics.

"Generally, we have got to be winning in Nottinghamshire, Derbyshire, Staffordshire, Lancashire, those types of places, because they contain a lot of the marginal constituencies that decide general elections.

"We want to make sure that we are turning out a strong Labour vote in Dudley, in Wolverhampton, in Walsall because they are key constituencies for us in a general election."

Ashworth also refuted claims that persuading Conservative voters to switch to Labour would be going against their principles.

"Some of the debate you see online, on Twitter and so on, suggests that if you want to get people who voted Conservative to switch to Labour, that is somehow a betrayal of our principles. It's absolutely not."

Jeremy Corbyn
Jeremy Corbyn has faced pressure as Labour leader after losing the Copeland constituency, held since 1935, in February. Reuters

Jeremy Corbyn has faced stiff opposition from Labour politicians since the Brexit vote, with several resigning from the Shadow Cabinet and others refusing to serve under him.

Ashworth moved away from suggestions he had regretted working with Corbyn, speaking to Sophy Ridge on Sunday, instead stressing he is "just someone who wants a Labour government" and who "wants the Labour party to succeed".

"We've got to win the trust of the British people. I'm not going to give up on that," Ashworth continued.

"I was offered a chance to take the government on and what they're doing to the NHS and to plot out a plan for the NHS for the future and that is what I'm doing."