The secretive practice has been going on since 2019, according to records obtained by The Telegraph and Liberty Investigates. The facial recognition searches were conducted even though policing minister Chris Philp did not mention the possibility of using the passport database until October 2023.

The majority of the biometric searches were conducted during the first nine months of last year when police authorities used the technology to trawl through passport images more than 300 times. The database holds records of 46 million British passport holders. Forces have also carried out searches of the UK immigration database, which holds information on foreign nationals, the investigation showed.

The Home Office has defended the practice, noting that the searches were conducted for the most serious offenses.

"This technology significantly helped progress numerous investigations including those for murder, rape and assault," the Metropolitan Police, which is responsible for almost one-third of passport searches, told The Telegraph.

This may not be the case for much longer. During his speech in October, Minister Philps suggested using the passport database to catch shoplifters. In December, it also emerged that a clause in the upcoming UK criminal justice bill could allow police to use facial recognition to search the database of Britain's 50 million driving licenses.

The new revelation has sparked concerns among members of the British parliament and watchdogs, with the office of Information Commissioner John Edwards promising to raise the issue with the Home Office. Non-governmental organizations which have been campaigning against the police use of facial recognition, such as Big Brother Watch, have also condemned the practice.