The children’s commissioner has launched a savage attack on the head of the NHS, accusing him of denigrating research that shows an “unacceptable” lack of children’s mental health provision.

In a highly unusual move, Anne Longfield has published an open letter to Simon Stevens, chief executive of NHS England, accusing him of ignoring young people’s experiences of the service and the frustrations of their parents. Laying out a list of grievances against him and his team, she also threatens to use the law to compel him to hand over data on waiting times for children’s mental health services.

Longfield made the decision to go public with her complaints – published on the commissioner’s website – after Stevens rubbished many of the claims in her recent report into children’s mental health, an issue she identified as her top priority after consulting with children.

“Many told me about their desperate attempts, sometimes lasting years, to access support, and even primary school children raised concerns about anxiety,” Longfield told Stevens in the letter. The report, published to coincide with World Mental Health Day last week, estimated that only between a quarter and a fifth of children with mental health conditions received help last year. It stated: “Progress in improving children’s mental health services has been unacceptably slow.”

Stevens is standing his ground. In a response to Longfield, obtained by the Observer, he suggests NHSE was “bounced” into giving a response to the report only after aspects of it were shared with journalists. He said that a key finding of the report, that “the government’s much-vaunted prioritisation of mental health has yet to translate into change at a local level”, was “demonstrably factually inaccurate”. Stevens writes: “I’m afraid we stand by our view that your report did indeed in places give a misleading view of NHS care.”

Longfield had warned that provision for young people was a postcode lottery and said that “children’s inability to access mental health support” was leading to a range of extra problems, “from school exclusions to care placements breaking down to children ending up in the youth justice system”.

It is highly unusual for the holders of two important public offices to be involved in such a public argument, particularly over an issue as sensitive as children’s mental health. Longfield, whose role is to promote and defend the rights of children, was appointed to the post by then education secretary Nicky Morgan in 2014. Stevens took his job at the head of NHS England (NHSE) in the same year.

Longfield’s report made difficult reading for the government at a time when concerns over the health service have reached a critical level. Theresa May identified improving mental health as a key issue but, before the busy winter period, there are concerns the NHS will struggle to cope with demand even for what are considered priority services, such as accident and emergency.

Stevens’s robust criticisms that the report contained “basic errors” were a powerful retort to Longfield’s claims. However, in a move that will ensure mental health provision for young people once again becomes a major political issue, Longfield has hit back, saying the report was published only after consultation with NHS England.

“The briefing was prepared using the NHS’s own data,” Longfield writes. “My conclusion on reading, checking and rechecking was that the service that exists at the moment is worryingly poor, a conclusion I stand by.”

In a highly personalised attack, Longfield tells Stevens: “I was very disappointed that NHS England’s response to our report, and your own personal response in front of the Commons health select committee, was to attempt to denigrate the research. Not once did you address the central issues raised. Instead, you and your team sought to undermine the important evidence that we are putting forward and strangely ignore the reality of children’s experiences of the service and the frustrations of their parents.”

Longfield suggested that Stevens’s claims that she and her team had not bothered to check the report were “untrue”. Similarly, claims that NHSE had not been given adequate time to review the report before commenting were also without foundation, according to Longfield. “I am under no duty or obligation to share my work in advance, yet we did so, out of courtesy.”

Stevens had indicated that NHSE was in possession of data that could be used to refute many of Longfield’s claims. However, the commissioner said: “If the NHS has data not in the public domain that disputes the picture we painted, then in the interests of transparency and accountability NHSE should publish it.”

She adds: “I hope you will feel able to share this data. As you will know, I am also able to demand such data under section 2f of the Children’s Act 2014.”

She concludes: “I hope in future we could all concentrate our efforts on tackling [health issues] rather than trying to undermine the work of those who make valid and constructive criticisms.”

Sarah Wollaston, chair of the health select committee, said the row would be discussed when the committee met on Tuesday.

“It’s very important that there is a constructive relationship between the Office of the Children’s Commissioner and NHS England,” she said.