is facing an exodus of staff quitting its ADHD and autism services to join private clinics amid huge demand from children and adults to get a diagnosis.

The loss of specialist personnel to the private sector has already forced one NHS trust in England to limit access to its neurodevelopmental service to only the most urgent cases.

The brain drain of psychologists, psychiatrists, nurses and other staff is leaving the NHS "weaker" and making it more likely that people will be forced to seek private healthcare.

The NHS care watchdog announced on Friday it will start inspecting private clinics that offer diagnostic assessments for ADHD and autism, which are currently unregulated. The Care Quality Commission's move follows the publication of the government-commissioned review of mental health and neurodiversity urging ministers to ensure all service providers are properly regulated amid claims of a "wild west" of private firms offering diagnostic tests.

Scores of private clinics have been set up in recent years offering assessments and treatment for ADHD and autism. They have capitalised on the large number of people who have been unable to access NHS care because services have been overwhelmed by an explosion in demand for diagnoses of neurodiversity, creating long waits for help and massive backlogs.

Adrian James, the CQC's interim inspector of mental health care, said: "This new responsibility will … close a significant gap in our oversight, meaning that people receiving these diagnoses can have far greater confidence in the service they are receiving." Prof Peter Fonagy's report – thought to be the biggest inquiry of its kind in the world to date – voiced concern that private firms face the temptation of "financial incentives" to wrongly diagnose and treat patients whose symptoms mean they should not qualify for either.

The CQC already regulates services offering diagnosis and treatment. But the change means non-NHS clinics offering only diagnostic assessments will in future have to with them.

Yvette Cooper, the health secretary, said the tighter regulation means "people can be confident that assessments are high quality and that the NHS is not being overcharged by independent or private providers". About 810,000 children and adults in England are waiting for an assessment.

The risks to the NHS of losing staff to the private sector was highlighted last week when the Health Service Journal revealed that the Herefordshire and Worcestershire health and care NHS trust had been forced to restrict children and young people's access to its neurodevelopmental service for that reason.

A paper presented to the trust's board reported: "Neurodevelopmental staffing is reduced significantly, with the majority of leavers choosing to take up employment with ISP [independent sector providers] and RtC [Right to Choose] providers for ND [neurodiversity]." Right to Choose gives NHS patients the right to have an NHS-funded ADHD or autism assessment with a private provider to help them receive care more quickly.

The trust responded to the exodus last month by limiting access by under-18s to only those with the most serious symptoms. It will revert to a normal service on Monday. A total of 1,298 people have already waited at least two years for an ADHD or autism assessment with the trust.

Thea Stein, the chief executive of the Nuffield Trust and a member of the government's ADHD taskforce, said NHS services suffered when staff joined private operators, as also happens in physiotherapy and child and adolescent mental health services.

She said: "Some staff stay with the NHS part-time when they get a private-sector job, but many will walk away entirely, leaving the health service in a weaker position and under greater pressure.

"Concerning inequalities will widen further if the NHS can't keep fully staffed teams or expand them to meet increasing demand for ADHD support. If these services continue to haemorrhage staff, some people will get much quicker support if they can afford to pay for private care," Stein added.

The NHS Alliance, which represents trusts, confirmed the continuing exodus of staff. "We're increasingly dealing with one ADHD workforce spread across different providers both in the NHS and private sector," Rebecca Gray, the director of the alliance's mental health network, said.

"The significant growth in the private sector market, where people accessing services can often be seen more quickly than in stretched NHS services, will make working in the independent sector attractive to some staff." The Independent Healthcare Providers Network (IHPN), a trade body that represents many non-NHS firms, welcomed the CQC's enhanced new role.

Sean Phillips, the IHPN's head of policy, said: "While all of IHPN members who deliver ADHD and autism assessments are CQCed – with 83% rated good or outstanding – we have long called for greater 'end to end' oversight of neurodevelopmental provision and it's welcome to see that the CQC have confirmed they will now be regulating all providers of ADHD and autism assessments."