Jeremy Corbyn is set to become the first official opposition leader to lose a by-election to the Government in 35 years, according to the Labour Party’s own canvass returns.

The Telegraph understands internal analysis of more than 10,000 conversations with voters in Copeland, the Cumbrian constituency, shows Labour’s support down by a third since 2015.

A senior Labour source said Mr Corbyn’s “incompetence” as a political leader is coming up repeatedly on the doorstep when discussing voting intention.

The returns suggest that the Tories will take Copeland – a seat held by Labour for 80 years – when voters pick their new MP next month.

It would be the first time the Government has won a seat off the official opposition since the Tories took Mitcham and Morden, London, in 1982.