The government’s failure to appoint an independent reviewer of its Prevent strategy and assess the controversial de-radicalisation programme’s effectiveness is to be challenged in court.

The decision by Rights Watch (UK) to initiate legal action against the Home Office comes amid terror attacks in London that raise questions about whether young men can be successfully directed away from terrorist violence.

Under the terms of the Counter-Terrorism and Border Security Act 2019, a report by an independent review of Prevent must be presented to parliament by mid-August.

Lord Carlile QC was appointed to conduct the review but stood down in December following objections over his previous strong support for the programme. No one has been chosen to replace him.

Rights Watch (UK) fears any new selection process will allow time only for a cursory review of the controversial programme and not be a rigorous examination of its aims or procedures.

Prevent has become a toxic brand for many within the Muslim community. By delaying the review, Rights Watch (UK) fears, the Home Office may be running down the clock so that it only delivers a “whitewash”.

Counter-terrorism police have placed the non-violent group Extinction Rebellion (XR) on a list of extremist ideologies that should be reported to the authorities running Prevent, raising further concerns about the focus of the programme.

The overwhelming majority of those referred to Prevent end up being dealt with by other agencies. Last year more of those given specialist support in what is known as the “channel” process were suspected of far-right radicalisation – involving 254 cases – than those suspected of Islamist radicalisation – 210 cases.

The stated aim of Prevent, which is a voluntary programme, is to divert people from terrorism before they offend. It deals with individuals who have yet to cross the criminality threshold.

Yasmine Ahmed, the director of Rights Watch, said “Prevent has involved a massively disproportionate number of people from Muslim communities.”

Part of the problem, she added, was because “the government has an incredibly vague definition of extremism which allows the government to catch a whole range of people who may be legitimately questioning government policy.

“There are a range of different de-radicalisation projects elsewhere, such as the Aarhaus model in Denmark that are more developed. We should look at how other countries are dealing with this.”

A letter before court action has been sent to the Home Office by Leigh Day, lawyers acting for Rights Watch (UK), which declares: “Parliament envisaged that the independent review would involve consultation with affected individuals and communities, as well as with the various civil society groups and organisations involved in the debate concerning the Prevent Strategy.

“It also envisaged a thorough and effective review of Prevent … and the concerns expressed about it. The delayed appointment of the independent reviewer reduces the likelihood that that will occur either at all, or with sufficient rigour.”

In response to an urgent parliamentary question last month by Diane Abbott, Labour’s home affairs spokesperson, the security minister, Brandon Lewis, said: “The statutory deadline for the review to be completed and its findings shared remains 12 August 2020. The next steps are being considered right now and will be announced in due course.”

The Home Office confirmed it had received the letter from Rights Watch (UK) and will respond.