Britain was not told of American plans to withdraw up to half its troops from Afghanistan, with officials only hearing of the abrupt shift by its closest military ally from news reports.

The decision caused surprise and unease among Western diplomats in Kabul, with one warning it was an early Christmas present to the Taliban.

Only hours before reports of the plan, the head of the British armed forces had said he believed Donald Trump would not announce a major troop withdrawal, as he had from Syria earlier this week.

Gen Sir Nick Carter, Chief of the Defence Staff, told the BBC he believed America was “wholeheartedly committed” to the international coalition propping up the Afghan government against a Taliban insurgency.

Mr Trump has reportedly ordered generals to begin withdrawing up to 7,000 of the 14,000 US troops currently helping the Afghans battle the Taliban and Islamic State group.

“ It was a surprise, we weren't expecting it. Now we need to see exactly what the detail is, before we see what we are doing.” said one British defence source.