The operation to evacuate the remaining British nationals and their local allies from Afghanistan is entering a “critical” phase, the head of the armed forces has warned.

General Sir Nick Carter, the chief of the defence staff, said they are working with the Taliban to ensure people can leave but he warned there are “a lot of challenges on the ground”.

His warning came as MPs were returning to Parliament from their summer break for an emergency sitting, three days after the Afghan capital Kabul fell to the militants.

Gen Sir Nick said he expects seven aircraft to head to Kabul, enabling another 1,000 people to leave on Wednesday.

He told BBC Radio 4’s Today programme: “There are a lot of desperate people trying to get to the airport, and subject to the situation remaining calm, which the Taliban are working hard to achieve alongside us, the system will work, we believe.

“At the moment we are collaborating with the Taliban on the ground, who are providing security.

“They are making sure that the centre of Kabul is very calm at the moment and so far we have not had reports of people finding it difficult to get to the airport.”

The UK Government has come under intense pressure over the handling of the downfall of the Western-backed government and the subsequent evacuation of British nationals and local allies.

On Tuesday night, Boris Johnson announced a new settlement scheme which will allow up to 20,000 Afghan vulnerable refugees to seek sanctuary in the UK over the coming years, with 5,000 over the next 12 months.

The PM is expected to tell MPs of the steps the international community needs to take to avoid a humanitarian catastrophe in Afghanistan, including the immediate increase in aid to the country and the surrounding region as well as a longer-term project to support refugees.

But the settlement scheme has been criticised as falling short of what is needed, and the PM can expect to come under fire from former armed services personnel on his own backbenches as he updates MPs on the work done to mitigate the crisis so far.

During a round of broadcast interviews, Home Secretary Priti Patel said it is important to have the right infrastructure and support in place before people are resettled.

“We cannot accommodate 20,000 people all in one go,” she told Sky News. “This is an enormous effort. We can’t do this on our own. We have to work together.”

She suggested the programme could eventually be expanded, telling BBC Breakfast: “We could end up bringing many more but first of all we have to have the underpinning and the infrastructure and the support to do that.”

Ms Patel said a separate scheme to resettle 5,000 interpreters and other local staff who had worked with the British was also being extended.

“There could be up to 10,000. We are expanding categories of people,” she told Sky News.

“We are working with the MoD (Ministry of Defence) on the ground. We are working with partners on the ground to identify these individuals.”

Speaking to US President Joe Biden on Tuesday night, Mr Johnson stressed the importance of work in the region and not to lose the gains of the last 20 years.

A No 10 spokesperson said: “The Prime Minister and President Biden agreed on the need for the global community to come together to prevent a humanitarian crisis in Afghanistan.”

But opposition parties said this was not enough and criticised the scope of the Afghan Citizens’ Resettlement Scheme, which will give priority to women and girls, and religious and other minorities.

This proposal does not meet the scale of the challenge

Human rights groups also hit out at Government plans over immigration more widely.

Nick Thomas-Symonds, Labour’s shadow home secretary, welcomed that a scheme was now in place but said there needed to be a “more urgent plan of action”.

He said: “This proposal does not meet the scale of the challenge. Not only does that risk leaving people in Afghanistan in deadly danger, it will also undermine the leadership role Britain must play in persuading international partners to live up to their responsibilities.”

Liberal Democrats leader Sir Ed Davey said the party would be calling on the Government to increase its uptake of refugees to 20,000 “over the next year”, telling BBC Breakfast: “We know it’s hundreds of thousands of people whose lives are at risk. Britain needs to play a part, with other countries, and we need to play a part urgently.”

Ian Blackford, the SNP’s leader in Westminster, said the target should be to welcome at least 35,000 to 40,000 Afghan refugees.

Since Saturday, officials said 520 British nationals, diplomats and former Afghan staff have left Afghanistan on UK military flights.

A flight carrying evacuated British nationals and Afghans landed at RAF Brize Norton in Oxfordshire at about 11pm on Tuesday night.

It comes after the Ministry of Defence said the first flight of British nationals and embassy staff arrived at the base on Sunday night.