Ministers are on course to miss their target of increasing the number of mental health staff by 21,000 by 2020, according to NHS workforce figures obtained by Labour.

A year after the government made the pledge, NHS mental health trusts in England had employed just 1,524 extra personnel, according to statistics collected by NHS Digital.

The very small rise is a setback for Theresa May’s plans to dramatically improve mental health care in order to reduce treatment delays, introduce new waiting times and reduce unmet need. Mental health chiefs and staff groups are worried that staffing problems will undermine those ambitions.

In July 2017 the then health secretary, Jeremy Hunt, said the recruitment of 21,000 extra personnel including psychiatrists, mental health nurses and therapists would help the NHS treat 1 million more people with mental health problems by 2020-21 and provide 24/7 care.

NHS Digital figures show that in August 2017 mental health trusts employed 179,333 staff. That had risen to 180,858 by August last year.

Barbara Keeley, the shadow mental health minister, who highlighted the statistics, said: “This government’s failure to act on the mental health workforce crisis could threaten to turn the burning injustice of mental ill-health that the prime minister pledged to tackle into a raging inferno.”

The disclosure of the modest increase comes days before the publication of the NHS long-term plan. In it NHS England will set out how it will use the extra money the prime minister has pledged to give it over the next five years, rising to £20.5bn more by 2023-24.

Improvements to mental health care over the next five to 10 years are expected to form a key part of the document. May has made mental health a priority in her two and a half years as prime minister, and promised that the long-term plan will increase mental health’s share of the NHS budget by at least £2bn by 2020-21.

The NHS Digital data also shows that if staff employed by two community services trusts in Liverpool and Staffordshire taken over by mental health trusts are included, the overall number of those working for the latter rose by 6,748 between August 2017 and August 2018.

However, that higher total includes about 5,000 staff that previously worked for the community trusts being reclassified as mental health staff as a result of the mergers.

A spokesperson for the Department of Health and Social Care said: “Mental health is a key priority for the government. We are transforming services with record amounts of funding, with the NHS spending almost £12bn on mental health in 2017-18.

“But we want to go further, which is why the prime minister has made parity between physical and mental health a priority for our long-term plan for the NHS supported by at least an additional £2bn a year.”