Jeremy Corbyn attended a conference after the Iraq war that called on Iraqis to engage in "military struggle" against coalition forces, the BBC's Panorama programme has learned.
Some 179 British soldiers, who were part of the coalition, died in Iraq.
Mr Corbyn, who is tipped to be the next Labour leader, attended the event on behalf of the Stop the War Coalition.
Mr Corbyn's team said the conference, in Cairo, had not been organised by Stop the War.
But Panorama has discovered the conference communiqué was posted on Stop the War's website and remained on it for many years.
The Cairo Conference was organised by an Egyptian anti-war organisation.
Its 2003 conclusions committed the conference to support "resistance against the occupation forces with all legitimate means, including military struggle".
At the time, Mr Corbyn was on Stop the War's steering committee.
Panorama asked Mr Corbyn whether he had supported the right of Iraqis to attack British soldiers - in line with the conference statement.
A spokesman for the Labour leadership candidate said: "Jeremy was opposed to the Iraq war and to the loss of 179 British soldiers. No lives should have been lost.
"His opposition to war was precisely because he did not wish to put British troops in harm's way, and voted inside Parliament against the war and worked with Stop The War (StWC) outside to stop it happening.
"As for the the Cairo conference, it was a separate organisation from StWC. Publication of its statement does not mean StWC endorsement of it.
"Still less did it mean that all the diverse membership of StWC steering committee agreed with it or were asked to endorse it."
Since he entered Labour's leadership race, Mr Corbyn's foreign policy views and activism have come under intense scrutiny.
Last month, he insisted remarks made in 2009 about Hezbollah and Hamas being his "friends" had been taken out of context.