British nationals of Afghan origin are being overlooked in the evacuation from Kabul, lawyers and campaigners have claimed as Dominic Raab said nearly all single-nationality UK citizens have been airlifted from the Taliban-controlled country.

About 50 British citizens, many with dual Afghan nationality, were understood to be outside Kabul airport on Wednesday. Some said they were waving their UK passports aloft in a desperate bid to flee as the deadline for western forces’ departure closed in.

Foreign Office officials had instructed them by email to register for evacuation at the Baron hotel outside the airport but several described scenes of chaos, with Afghan guards using electric cables on anyone who approached the door without being called and shots fired to prevent people getting too close to the gates.

Immigration lawyers raised concerns that these British citizens were being given second-class status in the emergency evacuation programme.

On Wednesday morning the foreign secretary confirmed that “almost all” single-nationality UK citizens who want to leave Afghanistan following the Taliban takeover have been brought home. “Mono-nationals – so single-nationality UK who have got documentation – the lion’s share, almost all of them that want to come out, have been brought home,” he told Sky News.

The number of dual national Britons remaining in the country is unclear. Raab said people awaiting evacuation “are rather complex cases, large family units where one or other may be documented or may be clearly a national, but it’s not clear whether the rest of them are”.

The apparent two-tier approach triggered anger. Sabir Zazai, the chief executive of the Scottish refugee council and a former Afghan refugee, said: “Britain has a duty to treat all its citizens equally but when it comes to this dreadful situation, these people who all have British citizenship are being treated differently. I appreciate that the [Foreign Office] staff are working hard here in the UK and on the ground in Kabul … but this is a failure of the UK government not planning and not preparing. These people’s lives are at risk.”

The treatment was discriminatory, he claimed. “There is a Windrush parallel here. If there were white men, women and children in the crowds, being shoved and pushed around, I think we’d have been very quick in getting them to safety,” he said.

Mohamad, a 32-year-old mechanic from Derby who has been a British citizen since 2012, said he had been waiting outside the gates for five days. “It’s horrible. The guards are shooting in the air, no one has been injured, but everyone is scared,” he said, speaking to the Guardian by phone from the queue.

Frustrated that so many British nationals were being refused assistance, he made a list on Tuesday of the names and UK passport numbers of citizens who were waiting by the hotel, hoping to hand it to officials inside. “No one listened to us. We weren’t allowed in to give the list to anyone. We had more than 40 names on that list, but I think there are at least 50.”

Although his wife does not have British citizenship, he had been told she was eligible for evacuation alongside him. “We’re holding our passports up in the air and waving them. We need help. We are all very scared,” he said.

Shabir, 31, who works for a food processing company in London and had been waiting outside the airport for three days and nights, said his was not a complex case. A British citizen for seven years, he no longer has an Afghan passport so classifies as a single-nationality.

He travelled to Afghanistan three months ago for the birth of his first child. His wife does not have a British passport but he was told by email to travel to the airport with her and their child. But he could not get close to the gate and was unable to let staff inside know that he was waiting. “We are feeling hopeless to be honest. There may just be 24 hours left. Are they going to leave British people behind?” he asked.

The shadow foreign secretary, Lisa Nandy, said she was seeking urgent assurances from the Foreign Office that all British citizens were being given equal priority. “The safe evacuation of UK citizens from Afghanistan must be an absolute priority … The government had 18 months to prepare for this moment. It is unconscionable that any British citizen should be left behind.”

The Foreign Office was asked for clarification on whether it is standard practice in evacuation operations to prioritise British nationals who hold only one passport. A Foreign Office worker processing applications inside Kabul airport said there was no hierarchy and that hundreds of applications an hour were being processed. “There are just so many people there’s no way to make that kind of hierarchy, we’re just trying to help as many people as possible,” the diplomat said. Staff were aware that “lots of Afghan Brits are unable to get through because of the chaos. It’s really difficult out there”.

Immigration lawyer Colin Yeo said: “If there are limited places and time, we can all understand the practical difficulties over whether non-British family members should be evacuated over British citizens. But there is no basis in British law for treating British citizens themselves any differently or worse because they also hold another nationality. Dominic Raab should retract his use of the discriminatory term ‘mono-nationals’.”

Toufique Hossain, public law director of Duncan Law, added: “It is difficult to see how these circumstances can be justified. It has a discriminatory effect on people who have equal status under British law. British citizens ought to be treated equally and with the same urgency even if they are also Afghan”.