Donald Trump plunged America’s Middle East policy into chaos on Wednesday as he declared the the Islamic State (Isil) had been “defeated” and ordered a complete withdrawal of US forces from Syria.

The shock decision to pull out US troops was immediately criticised by his own Republican allies as a “huge” mistake and his declaration of victory over Isil was openly contradicted by the British government.

Republicans warned the withdrawal would jeopardise the fight against Isil and undermine US hopes of countering Iran’s influence in Syria. It may also embolden Turkey to launch a major offensive against America's Kurdish allies in northern Syria.

US diplomats began an immediate evacuation from Syria within 24 hours. America’s 2,000 troops are expected to be all be withdrawn from the country by the end of March, US officials told Reuters.

“We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency,” Mr Trump wrote on Twitter.

Tobias Ellwood, a junior UK defence minister replied: “I strongly disagree. It has morphed into other forms of extremism and the threat is very much alive.”