NHS organisations are obliged by law to report people it fears at risk of becoming terrorists under the Prevent strategy - part of the UK government's counterterrorism plan aimed at stopping people becoming terrorists.
But new data collected by The BMJ has uncovered low levels of referrals to Prevent since the duty took hold, suggesting that it is having little impact in the NHS.
Figures obtained by freedom of information requests from 59 acute trusts in England found just 75 referrals to the programme between 2015-16. A further 74 trusts replied, but withheld information on referrals; some because the actual number of referrals was so small, others on security grounds.
Mental health trusts in England recorded a much higher level of referrals: 23 of these trusts (40%) were willing to provide data, showing a total of 254 referrals. This higher number is not surprising given that psychiatric patients are more likely to discuss such issues with their doctor.
The data also shows how many NHS staff have been left ill-prepared to deal with the Prevent programme. The BMJ asked trusts whether staff receive training, and responses from 134 acute trusts (81%) and 57 mental health trusts (95%) reveal a mixed picture.
For example, at St George's University Hospitals NHS Foundation Trust, south west London, just 4.5% have been trained, but at Barts Health NHS Trust, east London, 94% staff have undergone the Home Office-approved training.
A recent report by the US Open Society Justice Initiative warns of the threat the duty poses to the doctor-patient relationship and calls for its repeal in both the health and education sectors.
However, the GMC's updated confidentiality guidance makes it clear that in certain circumstances such as assisting in the prevention, detection or prosecution of serious crime, doctors would be "justified in disclosing information about patients in the public interest."
Other criticisms of Prevent is the lack of transparency about patients who have been referred and that the training offered to doctors and healthcare staff is limited.
The Royal College of Psychiatrists has also questioned the "variable quality of the evidence" underpinning the Prevent strategy and has called for it to be published in full.
In an exclusive interview, Metropolitan Police assistant commissioner Mark Rowley, the national lead for counterterrorism policing, tells The BMJ that the involvement of the NHS in Prevent is "massively important, more important than it's ever been."
Rowley claims that out of 13 attacks which the security services say they have foiled since 2013, "a disproportionate number of them do have mental health issues." He would not specify a number.
But he stresses that any suggestion mental illness causes terrorism is "complete nonsense" - and is adamant that collaboration between the NHS and the police is about safeguarding vulnerable people, not surveillance to identify criminals.
Rowley admits there needs to be more transparency around the strategy to win over sceptical clinicians - and acknowledges that research is racing to catch up with the change in terrorist tactics from networks to inciting lone attacks.
However, he points to two ongoing projects that aim to improve understanding of connections between mental health and vulnerability to radicalisation, and identify how specialist mental health teams should work with the police to reduce risk to the public and keep vulnerable people out of the criminal justice system.
"This is new and dynamic and we are going to have to learn together," he concludes.