Unions have demanded a meeting with Labour’s General Secretary over claims of a mole among party staff.

The allegation was made by Jeremy Corbyn’s closest adviser in a fly-on-the-wall documentary.

Seumas Milne, Mr Corbyn’s head of communications and strategy, is seen on camera complaining that questions prepared by the Labour leader and his team before Prime Minister's Questions leaked "a third of the time".

Mr Milne says: "This time they did (know the questions) because it leaked. It leaked from that meeting.

"It's very annoying because it only happens about a third of the time, but it obviously gives them (David Cameron’s team) a little bit of extra time.

"Whenever there is a leak it gives them that advantage - it gives them the advantage on TV as well."

The clip is included in Jeremy Corbyn:The Outsider, made by Vice News.

A source said that the party's General Secretary Iain McNicol was “furious” at the claim that a party staff member was a mole.

In a letter, seen by the Herald, Mr McNicol tells staff he wanted to write to them "personally" about the allegation.

He says that in his five years in the job he has “seen first-hand the dedication, commitment and professionalism displayed by Party staff on a daily basis”.

"Every day, including weekends and bank holidays, you work to put our values in to action and for that I am extremely grateful and in awe of your sheer ability to get things done.

"I wanted you to know, directly, that your work does not go unappreciated or unnoticed. A couple of conferences ago I said that we were a ‘movement on the march’. Every single one of you is an integral and vital part of that march.”

He adds that he knows that the matter “has upset a great many of you” and says that he plans to meet with Unite and GMB union representatives on the party's Joint Trade Union Committee to discuss the issue later today.

A source said that unions were "defending staff against this slur”.

Labour source said the 'mole' claim was "an aside".
"No one is pointing the finger at anyone and no one is blaming anyone."