George Galloway believes Jeremy Corbyn should have stuck to his principles and supported a motion calling for Tony Blair to be investigated for allegedly misleading parliament over the Iraq war.
Galloway made the comments during his talkRADIO show, while speaking to a Corbyn supporter who had called in.
The motion to investigate Blair was defeated in Parliament by a margin of 439 votes to 70 last week. Corbyn, a longstanding critic of Blair and the Iraq war, declined to participate in the debate or impose a three-line whip on his MPs.
Galloway said that Corbyn's principles were among his greatest strengths as a politician, and "he should very carefully consider whether behaving in an unprincipled way, whatever the odds, is the thing to do.
"He made, in my view, a very grave error of judgement in departing the House of Commons, leaving the field of battle, and not voting for the investigation into Tony Blair."
Galloway and his caller also discussed Labour First, a new organisation set up to represent moderate members of the Labour party and oppose Corbyn.
Listen to the interview above