Two police forces referred members of their own staff to a government programme designed to steer vulnerable individuals away from committing acts of terrorism, inspectors have revealed.

The revelation comes after a 21-year-old frontline Metropolitan police officer was arrested on suspicion of being a member of a banned rightwing terror group.

“Insider” threats posed by staff vulnerable to radicalisation are overlooked by the majority of forces, according to a report by Her Majesty’s Inspectorate of Constabulary and Fire & Rescue Services (HMICFRS).

The report also called into question the omission of the Ministry of Defence police (MDP) from the list of public sector bodies subject to the Prevent statutory duty, which legally compels workers to refer concerns about extremism to the programme.

The MDP is a civilian force that provides armed and unarmed security at sites around the country such as the Atomic Weapons Establishment at Aldermaston, housing estates for military personnel and training areas.

Overall, the inspectorate found that forces were meeting their Prevent duty with “many positive aspects of the Prevent work police forces do”.

Few police officers interviewed by inspectors recognised that colleagues may be vulnerable to radicalisation because of the influence of a partner or close family member, the report said.

“Only one force had arrangements in place acknowledging that members of staff vulnerable to radicalisation might need safeguarding,” it said. “The remainder would rely on existing counter-corruption or misconduct policies and monitoring by supervisors. Only two forces gave examples of having used a recognised Prevent referral approach to deal with a staff member who was vulnerable to extremism.

The vulnerability of staff generally was not referred to in forces’ Prevent training, the report said. “But it is a real threat. Few staff we interviewed had considered the possibility that their colleagues could be vulnerable to radicalisation.”

The constable arrested on 5 March on suspicion of membership of a proscribed organisation reportedly has alleged links to National Action, a neo-Nazi group. The constable was released on police bail until a date in late March pending further inquiries.

Last year, the Guardian revealed that a suspected far-right sympathiser was feared to be at large in the Met, after managing to scrawl, undetected, a swastika in a secure area of a police station. The culprit has not been caught and the discovery shocked minority ethnic staff and other employees. The swastika was found in February 2019, drawn on an inside wall at Edmonton police station in Enfield, north London, in an area only accessible to officers and staff.

Inspectors said the Home Office and the Ministry of Defence should review the MDP’s omission from schedule 6 to the Counter-terrorism and Security Act 2015. Schedule 6 lists public sector organisations and individuals subject to the Prevent statutory duty.

“Ministry of Defence police interviewees couldn’t understand why their force hadn’t been listed in schedule 6,” the report said. “At the time of the inspection, Ministry of Defence police were drafting a Prevent policy and standard operating procedure, as they believe they should feature in this legislation.

‘“Because of this, and similarities between this force and the Civil Nuclear Constabulary (and, to a lesser extent, British Transport Police), we believe the Prevent duty should apply.”

The report comes with the independent review of Prevent, a commitment to which was written into legislation, mired in crisis. The review’s former chair, Lord Carlile, was forced to step down in December over allegations regarding his lack of impartiality and the government is yet to appoint a replacement.

The total number of referrals to Prevent in the year to March 2019 was 5,738, the most recent available statistics show. However, only 10% went on to receive specialist support through what is known as the “Channel” process.

The National Police Chiefs’ Council lead for Prevent, Chief Constable Simon Cole, said: “Prevent is the only strategy that will succeed in reducing the terrorism threat in the long-term – and we must do more to champion its work and improve trust in our communities.

“As this report highlights, the police service can and will make improvements to the way we carry out our Prevent duties, and as the NPCC lead I will ensure every single one of the recommendations made by the HMICFRS will be met.”