The Labour deputy leader Tom Watson has warned that his party could be taken over by old Trots - Trotskyist entryists, who have no interest in winning elections but instead see Labour as a vehicle for revolutionary socialism.

But what is a Trot, who was Leon Trotsky and what do his followers in Britain actually want?

David Aaronovitch is joined by:

Prof John Callaghan, Salford University

Amy Leather of the Socialist Workers Party

Michael Crick author of 'The March of Militant' and a journalist with Channel 4 News

Dr Bert Patenaude, Stanford University, author of 'Trotsky: Downfall of a Revolutionary'

Producer: Joe Kent