Labour deputy leader Tom Watson has been accused of being behind a “dirty tricks” campaign to try to oust Jeremy Corbyn's biggest union backer Len McCluskey.

The row erupted after Sion Simon, Labour's candidate in the West Midlands mayoral election, allowed his call centre to be used to telephone party members to see if they will support Gerard Coyne, Mr McCluskey's rival in the Unite election campaign.

Unite said this appeared to be a breach of data protection laws and has now threatened to report it to the information watchdog which has the power to levy large fines.

In correspondence with Unite, which has been seen by The Telegraph, Labour admitted it was not allowed to share personal information about its members with third party campaigns but denied breaching any laws.

Mr Simon is a close ally of the party's deputy leader Tom Watson who has had a very public and high profile falling out with Mr McCluskey.

Sources close to Mr McCluskey said the report was evidence of a “dirty tricks” campaign to try to stop him winning another five years as Unite leader.