Tony Blair, the British prime minister, was on Friday forced to confront an unexpected and unprecedented challenge to his Iraq policy when the head of the British army said troops must leave the country “soon” because their presence there “exacerbates the security problems”.

In public statements, the prime minister has always insisted that UK troops would withdraw only once Iraqi security forces were fully trained. Just two weeks ago, Mr Blair said: “If we retreat now...we will be committing a craven act of surrender that will put our future security in the deepest peril.”