Women, children and religious minorities will be prioritised in a new UK resettlement scheme for 20,000 Afghan refugees, Boris Johnson will announce, acknowledging that those who helped the western coalition over two decades are now most at risk from the Taliban takeover.

But most of the 20,000 are likely to have fled to neighbouring countries such as Pakistan before being resettled in Britain over five years, a Whitehall source said, unless the UK can strike an agreement with the Taliban to let people depart.