Theresa May has dodged a key question on whether she would protect extra money earmarked for children's mental health at the same time as she pledged to “transform” support for young people.

The Prime Minister is facing renewed calls to prevent health commissioners from spending hundreds of millions of pounds on other priorities instead of child and adolescent mental health services, on the same day she announced trials of new services in schools and workplaces.

However, when asked at a press conference whether she would "forcibly" ring-fence an extra £1.25bn of funding announced by George Osborne in 2015, she swerved the question and moved on to a more general commitment on NHS funding. "We asked the NHS to produce their own five-year plan, which they did. They said it would require £8bn of extra spending, we've actually put £10bn of extra spending in. So I recognise the pressures that exist in the NHS," she said.

Research by the Young Minds charity found that nearly two-thirds of clinical commissioning groups (CCGs) – the GP-led bodies that buy in health services across the country – used some or all of the extra cash promised by Mr Osborne to "backfill cuts or to spend on other priorities" in 2015-16.

Norman Lamb, the former Liberal Democrat care minister, said it amounted to "theft" and called for the money to be protected.

He told Sky News: "There has to be a mechanism to ensure that when the Chancellor of this country stands up and makes a budget speech, and makes a commitment to extra investment in children's mental health, that the money is spent where it's intended."

The Department of Health said it did not ring-fence budgets as a matter of policy, and held NHS England to account over whether money was spent in accordance with its own five-year forward plan.

Dr Phil Moore, a GP and chairman of the Mental Health Commissioners Network, told The Independent: "It seems that some CCGs have felt they have been unable to invest that [money] in children's and young people's services because of the pressures on their funding.

"We would like to strengthen the commissioners' arms by ring-fencing that money."

The Department of Health "did not feel that they could do that", he said, while commissioners had to tackle the dilemma of "this money has come, it's not ring-fenced, so what are the biggest needs of our local population?".

He added: "The funding is in dire straits, period. The providers and commissioners are all having to deal with that.

"I think children and young people is a huge priority."

Ms May's announcement was welcomed this morning by Professor Sir Simon Wessely, the chairman of the Royal College of Psychiatrists, who said he saw "good signs in her speech that she is intending to take this seriously over the long term".

He added: "The Prime Minister needs to take a broad view. She needs to focus on how do we build up an enthusiastic workforce. You need to be continually renewing."