The first charter flight taking British nationals back to the UK from the Middle East has taken off as the prime minister described the ongoing evacuation operation as one of the biggest of its kind.

Keir Starmer announced that the delayed plane from Oman, which was originally scheduled to leave at 7pm on Wednesday, had taken off minutes before he addressed a Downing Street press conference.

The airlift of British nationals takes place against the backdrop of continued US and Israeli strikes on Iran, with missiles and drones also raining down on other states in the region.

More than 2,000 people had arrived in the UK on Wednesday onboard eight flights from the United Arab Emirates. A further 10 flights from the UAE to the UK are expected on Thursday.

Starmer said on Thursday: “I want to be very clear, this is a huge undertaking. It’s one of the biggest operations of its kind, many times bigger than the evacuation from Afghanistan. It’s not going to happen overnight, but we will not stop until our people are safe.”

However, the UK government has continued to come under pressure from MPs, as well as from British nationals in the Middle East, who have criticised the pace and scale of the evacuation effort.

A British person onboard the charter flight from Oman’s capital of Muscat included a passenger named Mark, who described the situation on Wednesday night as a “complete shambles” involving “loads of issues with check-in” and “no communication from the craft or the crew” while being stuck on a transfer bus.

In parliament, the Conservative leader pressed the government on how long it would take to get British nationals to safety, while the prominent Labour backbencher and foreign affairs committee chair, Emily Thornberry, passed on the concerns of constituents caught up at Muscat airport who said they had difficulty identifying British consular staff.

British Airways has meanwhile said it would run a fourth daily flight from Muscat to London Heathrow – a route it does not usually serve – departing at 10.30pm on Saturday.

More than 140,000 Britons have registered their presence in the Middle East with the Foreign Office, a minister in the department, Hamish Falconer, told parliament. Bringing UK nationals home was “an enormous exercise and ministers must be honest about all their actions.” he was told by the shadow foreign secretary, Priti Patel.

Falconer, a former diplomat with frontline experience of previous evacuations in other crises, described the situation as “a consular challenge on a scale not seen since Covid,” adding that there were “no instant solutions”.

British nationals in Oman will be contacted as soon as the additional government-organised flights from Muscat become available, he said, while adding that commercial flights were the most likely and the most rapid routes back to the UK.

Some 138,000 British nationals in the Gulf – including more than 100,000 who are in the United Arab Emirates – have registered their presence to the Foreign Office. Those who qualify to leave on government flights are being asked to pay for seats while the most vulnerable people are being prioritised.

The first government rescue flight from the Middle East, which failed to take off from Oman, was delayed because of problems “getting passengers on board”, Home Office minister Alex Norris said in a media interview on LBC radio.