The shadow chancellor has said the decision to bomb Syria is a "shameful step backwards" and warned it could foster another generation of militants determined to commit a Paris-style terror attack.
John McDonnell said the air strikes launched this week would result in the loss of lives and could make Britain less safe.
He reiterated his comments about Hilary Benn's speech in favour of the strikes against Islamic State (IS) - warning that the finest oratory can lead to the greatest mistakes.
Addressing The People's Assembly conference in central London, Mr McDonnell said: "I have to say this week was a disgraceful, shameful step backwards. And I find it appalling."
He said a political solution should have been pursued rather than a bombing campaign.