The government has stripped 21 Britons of their British passports in the last year because they support terrorist networks, such as the Islamic State, also known as ISIS.
The most well-known example is that of radical cleric Anjem Choudary, whose passport was stripped in September because officials believed he was preparing to live under the rule of the Islamic State. But details of how many citizens have had their passport taken away was not previously in the public domain.
Under the Royal Prerogative power, the Home Office can either refuse to give citizens passports, or withdraw them entirely, if their having a passport could contravene the public interest, for example if they are "seeking to harm the UK or its allies".
It is believed that the government is aware of a number of citizens who support terrorist organisations in Syria in Iraq, such as ISIS.
The defence secretary Philip Hammond last month suggested that any Britons who travel to join the Islamic State could be tried for "high treason" if they return to the UK.
The numbers were revealed by Lord Bates, parliamentary undersecretary to the home office, in response to a parliamentary question.
He wrote: "From November 2013 to 11 November 2014, the Royal Prerogative power to withdraw or refuse a British Passport has been exercised 21 times on public interest grounds against British citizens thought to be involved in the support of ISIL and other extremist groups in Syria and Iraq."