The Government has secretly ramped up a controversial programme that strips people of their British citizenship on national security grounds – with two of the men subsequently killed by American drone attacks.

An investigation by the Bureau of Investigative Journalism for The Independent has established that since 2010, the Home Secretary, Theresa May, has revoked the passports of 16 individuals, many of whom are alleged to have had links to militant or terrorist groups.

Critics of the programme warn that it allows ministers to “wash their hands” of British nationals suspected of terrorism who could be subject to torture and illegal detention abroad.

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They add that it also allows those stripped of their citizenship to be killed or “rendered” without any onus on the British Government to intervene.

At least five of those deprived of their UK nationality by the Coalition were born in Britain, and one man had lived in the country for almost 50 years. Those affected have their passports cancelled, and lose their right to enter the UK – making it very difficult to appeal against the Home Secretary’s decision. Last night the Liberal Democrats’ deputy leader Simon Hughes said he was writing to Ms May to call for an urgent review into how the law was being implemented.

The leading human rights lawyer Gareth Peirce said the present situation “smacked of mediaeval exile, just as cruel and just as arbitrary”.

Ian Macdonald QC, the president of the Immigration Law Practitioners’ Association, described the citizenship orders as “sinister”.

“They’re using executive powers and I think they’re using them quite wrongly,” he said. “It’s not open government; it’s closed, and it needs to be exposed.”

Laws were passed in 2002 enabling the Home Secretary to remove the citizenship of any dual nationals who had done something “seriously prejudicial” to the UK, but the power had rarely been used before the current government took office.

The Bureau’s investigation has established the identities of all but four of the 21 British passport holders who have lost their citizenship, and their subsequent fates. Only two have successfully appealed – one of whom has since been extradited to the US.

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In many cases those involved cannot be named because of ongoing legal action. The Bureau has also found evidence that government officials act when people are out of the country – on two occasions while on holiday – before cancelling passports and revoking citizenships.

Those targeted include Bilal al-Berjawi, a British-Lebanese citizen who came to the UK as a baby and grew up in London, but left for Somalia in 2009 with his close friend the British-born Mohamed Sakr, who also held Egyptian nationality.

Both had been the subject of extensive surveillance by British intelligence, with the security services concerned they were involved in terrorist activities.

Once in Somalia, the two reportedly became involved with al-Shabaab, the Islamist militant group with links to al-Qa’ida. Mr Berjawi was said to have risen to a senior position in the organisation, with Mr Sakr his “right-hand man”.

In 2010, Theresa May stripped both men of their British nationalities and they soon became targets in an ultimately lethal US manhunt.

In June 2011 Mr Berjawi was wounded in the first known US drone strike in Somalia and last year he was killed by a drone strike – within hours of calling his wife in London to congratulate her on the birth of their first son.

His family have claimed that US forces were able to pinpoint his location by monitoring the call he made to his wife in the UK. Mr Sakr, too, was killed in a US airstrike in February 2012, although his British origins have not been revealed until now.

Mr Sakr’s former UK solicitor said there appeared to be a link between the Home Secretary removing citizenships and subsequent US actions.

“It appears that the process of deprivation of citizenship made it easier for the US to then designate Mr Sakr as an enemy combatant, to whom the UK owes no responsibility whatsoever,” Saghir Hussain said.

Mr Macdonald added that depriving people of their citizenship “means that the British government can completely wash their hands if the security services give information to the Americans who use their drones to track someone and kill them.”

The campaign group CagePrisoners is in touch with many families of those affected. Its executive director Asim Qureshi said the Bureau’s findings were deeply troubling for Britons from an ethnic minority background.

“We all feel just as British as everybody else, and yet just because our parents came from another country, we can be subjected to an arbitrary process where we are no longer members of this country any more,” he said.

“I think that’s extremely dangerous because it will speak to people’s fears about how they’re viewed by their own government, especially when they come from certain areas of the world.”

The Liberal Democrat deputy leader Simon Hughes said that, while he accepted there were often real security concerns, he was worried that those who were innocent of Home Office charges against them and were trying to appeal risked finding themselves in a “political and constitutional limbo”.

“There was clearly always a risk when the law was changed seven years ago that the executive could act to take citizenship away in circumstances that were more frequent or more extensive than those envisaged by ministers at the time,” he said.

“I’m concerned at the growing number of people who appear to have lost their right to citizenship. I plan to write to the Home Secretary and the Home Affairs Select Committee to ask for their assessment of the situation, and for a review of whether the act is working as intended.”

Ms Peirce, a leading immigration defence lawyer, said, “British citizens are being banished from their own country, being stripped of a core part of their identity yet without a single word of explanation of why they have been singled out and dubbed a risk,” she said.

Families are sometimes affected by the Home Secretary’s decisions. Parents may have to choose whether their British children remain in the UK, or join their father in exile abroad.

In a case known only as L1, a Sudanese-British man took his four British children on holiday to Sudan, along with his wife, who had limited leave to remain in the UK. Four days after his departure, Theresa May decided to strip him of his citizenship.

With their father excluded from the UK and their mother’s lack of permanent right to remain, the order effectively blocks the children from growing up in Britain. At the time of the order the children were aged between eight and 13 months.

The judge, despite recognising their right to be brought up in Britain, ruled that the grounds on which their father’s citizenship was revoked “outweighed” the rights of the children.

Mr Justice Mitting, sitting in the semi-secret Special Immigration Appeals Commission (Siac), said: “We accept it is unlikely to be in the best interests of the appellant’s children that he should be deprived of his British citizenship...

“They are British citizens, with a right of abode in the UK.

“They are of an age when that right cannot, in practice, be enjoyed if both of their parents cannot return to the United Kingdom.”

Yet he added that Theresa May was “unlikely to have made that decision without substantial and plausible grounds”.

In another case, a man born in Newcastle in 1963 and three of his London-born sons all lost their citizenship two years ago while in Pakistan.

An expert witness told Siac that those in the family’s situation may be at risk from the country’s government agencies and militant groups. Yet Siac recently ruled that the UK “owed no obligation” to those at risk of “any subsequent act of the Pakistani state or of non-state actors [militant groups] in Pakistan”.

The mother, herself a naturalised British citizen, now wants to return here in the interests of her youngest son, who has developmental needs. Although 15, he is said to be “dependent upon [his mother and father] for emotional and practical support”.

His mother claimed he “has no hope of education in Pakistan’. But the mother has diabetes and mobility problems that mean she “does not feel able to return on her own, with or without [her son].”

Mr Justice Mitting ruled that the deprivation of citizenship of the family’s father had “undoubtedly had an impact on the private and family life of his wife and youngest son, both of whom remain British citizens”. But he added that the father posed such a threat to national security that the “unavoidable incidental impact” on his wife and youngest son was “justifiable”, and dismissed the appeal.

A Home Office spokeswoman said: “Citizenship is a privilege not a right. The Home Secretary has the power to remove citizenship from individuals where she considers it is conducive to the public good. An individual subject to deprivation can appeal to the courts.”

She added: “We don’t routinely comment on individual deprivation cases.”

Asked whether intelligence was provided to foreign governments, she said: “We don’t comment on intelligence issues. Drone strikes are a matter for the states concerned.”

Mahdi Hashi: From Camden care worker to US prisoner

Mahdi Hashi, a former care worker from Camden in north London, was well known to Britain’s security services – in fact they tried to recruit him when he was 19.

Now 23, Mr Hashi is in a high-security US prison having been secretly “rendered” from the African state of Djibouti last year.

Mr Hashi claims that before being sent to the US on charges of working with the terrorist group al-Shabaab he witnessed torture in an African prison, before being handed over to the CIA and forced to sign a confession.

Despite Mr Hashi being brought up in the UK, the British Government has washed its hands of him, having stripped him of his citizenship shortly before he disappeared in Somalia last summer.

His UK family say that when they lost contact with their son they approached the Foreign Office for help. But they were told by officials that they could not provide assistance because the Home Secretary had issued an order depriving him of his British citizenship.

It was only five months later, when he re-appeared in the US, that they were able to contact him again. The family’s lawyer, Saghir Hussain, said at the time: “The UK Government has a lot of explaining to do. What role did it play in getting him kidnapped, held in secret detention and renditioned to the US?”

The case has led to allegations that Britain may have conspired with the US to strip Mr Hashi of his citizenship knowing he would be arrested in Africa. They have no further obligations towards him and can avoid potentially embarrassing questions about his treatment before his rendition.

The case is all the more bizarre as Mr Hashi gave an interview to The Independent in 2009 when he alleged that MI5 had attempted to recruit him. He claimed that on a previous trip to Africa he was held for 16 hours in a cell at Djibouti airport, and that when he was returned to the UK he was met by an MI5 agent who told him his terror-suspect status would remain until he agreed to work for the Security Service. He alleges he was to be given the job of informing on his friends by encouraging them to talk about jihad.

Mohamed Sakr: The British car valet killed by a drone strike

In February last year, international agencies in Africa reported that “four foreign Islamist militants” had been killed in a drone strike south of Somalia’s capital, a day after the country’s Prime Minister called for foreign air strikes against the terror group al-Shabaab.

At the time a senior Western intelligence officer was quoted as saying that a “very senior Egyptian was killed” in the raid, along with three Kenyans and a Somali.

That was technically true – but in reality the Egyptian had not even been born in the country for which he held a passport. It would have been more accurate to describe him as a British terror suspect who once ran a car valeting business in London.

The Bureau has established that the victim of the February air strike was Mohamed Sakr, who was born and brought up in the UK before having his citizenship revoked in September 2010 by the Home Secretary, Theresa May.

Sakr appears to have come to the attention of UK intelligence officials after he visited Saudi Arabia, Egypt and Dubai in 2007. He was then repeatedly targeted by counter-terrorism officers over a two-year period, according to reports.

It was this, it is alleged, that drove Sakr out of the country; he left Britain in late 2009 for Somalia.

The law allowing the Home Secretary to remove citizenship was in place when Sakr left the UK, but it was not until after the Coalition came to power that it was used in his case.

It would be another year and a half before he was killed.