General Sir Michael Walker, chief of the defence of staff, also said, in an interview with the Sunday Times, that Britain and the United States will have to make do with a less-than-perfect outcome from the US-led war.

The March 2003 invasion of Iraq to topple Saddam Hussein initially helped to attract new recruits and lift morale in the armed forces, despite being unpopular among the British public, said Walker.

"There was an understanding by members of the armed services that this was not an all-hands-up, popular event across the country," he said.

"But I think at that stage they were able to decouple in their own minds, as I was, the fact that the country was not necessarily behind the strategic decision to go to war, but once our boys and girls were out there, doing their various things, they would support them in that role," Walker said.