Ian Lavery, the Labour chairman, left, is backing Rebecca Long Bailey in the leadership contest

Senior allies of Jeremy Corbyn have called for northern working class voters to join Labour in a move seen by party members as an attempt to block Sir Keir Starmer’s run for the leadership.

Ian Lavery, Labour’s chairman, and Jon Trickett, a shadow cabinet office minister, have urged supporters from outside southern cities to sign up to “rebalance” the party. Members who join before January 20 will be able to vote in the race to replace Mr Corbyn.

The Labour leader announced that he was standing down after the general election, at which the party suffered its worst defeat since 1935. Mr Corbyn remains in post until his successor is announced in April.

Labour has also opened a two-day window for members