The government has refused to reveal how many people have been deprived of their British citizenship in the past two years after dramatically increasing its use of controversial powers to prevent the return of Isis members.

The number of people subjected to the measure rose by more than 600 per cent in 2017, despite an official review warning that it might be an “ineffective and counter-productive weapon against terrorism”.

An official “transparency” report containing statistics on citizenship deprivations was due for release last summer but has not yet been published, and the Home Office refused to give The Independent updated figures.

Officials would not give a reason for the delay, meaning that no new information has been released for 18 months.

The situation is no clearer in Syrian prisons holding Isis members, where British inmates interviewed recently by The Independent said they had not been told whether they were still UK citizens.

Timeline of the Isis caliphate Show all 19 1 /19 Timeline of the Isis caliphate Timeline of the Isis caliphate ISIS began as a group by the merging of extremist organisations ISI and al-Nusra in 2013. Following clashes, Syrian rebels captured the ISIS headquarters in Aleppo in January 2014 (pictured) AFP/Getty Timeline of the Isis caliphate Abu Bakr Al-Baghdadi declared the creation of a caliphate in Mosul on 27 June 2014 Timeline of the Isis caliphate Isis conquered the Kurdish towns of Sinjar and Zumar in August 2014, forcing thousands of civilians to flee their homes. Pictured are a group of Yazidi Kurds who have fled Rex Timeline of the Isis caliphate On September 2 2014 Isis released a video depicting the beheading of US journalist Steven Sotloff. On September 13 they released another video showing the execution of British aid worker David Haines Timeline of the Isis caliphate The US launched its first airstrikes against Isis in Syria on 23 September 2014. Here Lt Gen William C Mayville Jnr speaks about the bombing campaign in the wake of the first strikes Getty Timeline of the Isis caliphate Isis militants sit atop a hill planted with their flag in the Syrian town of Kobani on 6 October 2014. They had been advancing on Kobani since mid-September and by now was in control of the city’s entrance and exit points AFP/Getty Timeline of the Isis caliphate Residents of the border village of Alizar keep guard day and night as they wait in fear of mortar fire from Isis who have occupied the nearby city of Kobani Getty Timeline of the Isis caliphate Smoke rises following a US airstrike on Kobani, 28 October 2014 AFP/Getty Timeline of the Isis caliphate YPG fighters raise a flag as they reclaim Kobani on 26 January 2015 VOA Timeline of the Isis caliphate Isis seized the ancient Syrian city of Palmyra on 20 May 2015. This image show the city from above days after its capture by Isis Getty Timeline of the Isis caliphate Kurdish forces are stationed on a hill above the town of Sinjar as smoke rises following US airstrikes on 12 November 2015 AFP/Getty Timeline of the Isis caliphate Kurdish forces enter Sinjar after seizing it from Isis control on 13 November 2015 AFP/Getty Timeline of the Isis caliphate Iraqi government forces make the victory sign as they retake the city of Fallujah from ISIS on 26 June 2016 Getty Timeline of the Isis caliphate Iraqi forces battle with Isis for the city of Mosul on 30 June 2017 AFP/Getty Timeline of the Isis caliphate Members of the Iraqi federal police raise flags in Mosul on 8 July 2017. On the following day, Iraqi prime minister Haider Al Abadi declares victory over Isis in Mosul Getty Timeline of the Isis caliphate Members of Syrian Democratic Forces celebrate in Al-Naim square after taking back the city of Raqqa from Isis. US-backed Syrian forces declare victory over Isis in Raqqa on 20 October 2017 after a four-month long campaign Getty Timeline of the Isis caliphate Female fighters of the Syrian Democratic Forces celebrate in Al-Naim Square after taking back the city of Raqqa from Isis. US-backed Syrian forces declare victory over Isis in Raqqa on 20 October 2017 after a four-month long campaign AFP/Getty Timeline of the Isis caliphate Trucks full of women and children arrive from the last Isis-held areas in Deir ez-Zor, Syria in January 2019 They were among the last civilians to be living in the ISIS caliphate, by this time reduced to just two small villages in Syria’s Deir ez-Zor Richard Hall/The Independent Timeline of the Isis caliphate Zikia Ibrahim, 28, with her two-year-old son and 8-month-old daughter, after fleeing the Isis caliphate, on Saturday 26 January 2019 Richard Hall/The Independent

Some asked journalists to request information from the government on their behalf, while others found out through media reports that they had been stripped of British nationality.

Campaigners said the government’s silence suggested that it has “something to hide”, amid legal challenges and allegations that it has broken international law by making people stateless.

After teenage Isis supporter Shamima Begum – who this week lost the first stage of a legal challenge against the decision to revoke her British citizenship – was found in Syria last year, the then home secretary, Sajid Javid, said she was one of about 150 people to be deprived of their citizenship since 2010.

However, that figure only includes the total until the end of 2017, which was published in July 2018 as part of the government’s most recent Transparency Report on Disruptive and Investigatory Powers.

The document showed that citizenship deprivations were used only a handful of times a year, until rocketing from 14 in 2016 to 104 in 2017.

It said the home secretary can apply the measure “if satisfied that such action is ‘conducive to the public good’ or if the individual obtained their British citizenship by means of fraud, false representation or concealment of material fact.

“Deprivation is particularly important in helping prevent the return to the UK of certain dual-national British citizens involved in terrorism-related activity in Syria or Iraq.”

Critics have compared the power to “exile” and questioned both its legal basis and effectiveness.

The Liberty campaign group said there was “no evidence” that increasing the use of citizenship deprivation makes the British public safer.

“The government’s silence on this subject suggests it has something to hide,” its advocacy director, Clare Collier, told The Independent.

Shamima Begum's child could retain British citizenship, admits Sajid Javid

“Stripping people of their British citizenship is exile in all but name, and is one of many powers the government has expanded in the name of counterterrorism in recent years.

“This is despite the fact there is no evidence it keeps us safe. In fact, using mediaeval powers like exile in lieu of proper investigation and prosecution erodes the rule of law and makes us less safe.

“Ministers must reveal how many people have been stripped of British citizenship in the last two years.”

There is disquiet in governmental circles over the delay to the transparency report, which has not been explained to civil servants.

It is one of several reports relating to security matters that have been written and submitted to the government but withheld from publication.

The lengthy document contains information on the use of powers including Terrorism Prevention and Investigation Measures (Tpims), deportations, asset freezing, covert surveillance and the interception of electronic communications.

Jonathan Hall QC, the Independent Reviewer of Terrorism Legislation, said the report was also the only official source of data about Temporary Exclusion Orders.

“The media and the public should not have to wait until the publication of my own report, which is still outstanding, before seeing how frequently this important counter-terrorism power is used,” he added.

The total number of British jihadis remaining in Syria or Iraq is unknown. According to government figures, more than 900 people have travelled from the UK to join Isis and other jihadi groups in the countries since 2014.

At least 20 per cent of them have been killed in the region and around 40 per cent have already returned.

Pleas by the Syrian Defence Forces (SDF) for the UK to repatriate jihadi fighters have fallen on deaf ears, and the group has warned it will not be able to hold them indefinitely.

A government-commissioned review warned in 2016 that removing extremists’ citizenship left them free to continue terrorist activity abroad, prevented monitoring and encouraged the “dangerous delusion that terrorism can be made into a foreign problem”.

“The power has been characterised as an ineffective and counter-productive weapon against terrorism,” said a report by Lord Anderson, then Independent Reviewer of Terrorism Legislation.

It quoted research saying that the measure amounted to “a policy of catch and release, setting up today’s convicts as tomorrow’s foreign fighters” and exporting them to places where they could not be monitored.

Last year, the former defence minister Tobias Ellwood told The Independent that the British government might be creating the conditions for an Isis resurgence by leaving fighters in Syrian prisons.

“We’ll see Daesh 2.0,” he warned, using another name for Isis. “They will regroup to fight another day – we’re already seeing it.”

The government has vowed to use any power available to prevent jihadis returning to the UK, and claimed that citizenship deprivation was used in order to protect Britain.

A Home Office spokesperson said: “Depriving a dual national of citizenship is just one way we can counter the terrorist threat posed by some of the most dangerous individuals and keep our country safe.