Any extension to the deadline for withdrawal from Afghanistan would cross a red line that will lead to "consequences", the Taliban has warned.

       The US had set August 31 as the end point, to ensure the US presence had ended before the anniversary of September 11.

       But Boris Johnson is planning to personally appeal to Joe Biden for a delay to the withdrawal of US forces from Afghanistan, after the chaos caused by the short timeline led to seven people being killed in the crush at Kabul airport.

       The Prime Minister will use an emergency G7 meeting on Tuesday to press the case for a delay, after attempts by both Foreign Secretary Dominic Raab and Defence Secretary Ben Wallace to ask the same of their US counterparts fell on deaf ears.

       However, Taliban spokesperson Dr Suhail Shaheen told Sky News: "It's a red line. President Biden announced that on 31 August they would withdraw all their military forces. So if they extend it that means they are extending occupation while there is no need for that."

       He added: "If the US or UK were to seek additional time to continue evacuations - the answer is no. Or there would be consequences. It will create mistrust between us. If they are intent on continuing the occupation it will provoke a reaction."

       James Heappey, a defence minister, said that as a former soldier he had a "deeply ingrained" belief that people should not be left behind, but acknowledged the "hard reality" that the current conditions meant that is precisely what will happen.

       "No one has made any secret of the fact that we would have preferred a more conditions-based withdrawal, but Nato agreed collectively that we went in together and should come out together," he told Sky News.

       He reiterated that the UK's presence was "fundamentally underpinned" by the US presence, adding: "Whether or not the US can be persuaded to stay is a matter for the Prime Minister tomorrow in the G7 meeting after the initial overtures made by both the Foreign Secretary and the Defence Secretary in the days previous, but the conversation with the Taliban will then follow."

       Mr Heappey added: "We have been having these conversations with Americans, but this isn’t just a decision that is made alone in Washington - the Taliban get a vote on this too, and it is far from certain that they are willing to accept an extension."

       A former US Director of National Intelligence has warned that unless the deadline was pushed back, many people would be left behind.

       James Clapper, who served under Barack Obama, told BBC Radio 4's Today programme: "What I hear is the deadline and then the assurance that we are doing everything they can to get our citizens out and I assume our allies' citizens out, as well as the Afghans who assisted us. I am hearing both, so I really think in practice if we have to go beyond August 31 we probably will.

       "That is somewhat of an arbitrary deadline," he added. "The objective originally was to get everybody out before 9/11- well, we may not be able to do that now."