New waiting-time targets for mental health treatment and stronger support for young adults with difficulties will be an important part of the government’s long-awaited 10-year NHS plan to be launched on Monday.

“Comprehensive access standards” for mental health will be introduced for the first time, in an effort to close the gap between physical and mental health treatment. Waiting-time targets are well established for A&E treatment, surgery and cancer care.

The plan will include an extra £2.3bn to pay for talking therapies for an additional 350,000 young people and 380,000 adults over the next five years. An around-the-clock helpline will eventually be established. Young people will no longer be forced to restart their treatment with adult services when they turn 18.

The plan will also overhaul maternity care and improve staffing levels. It involves spending an extra £20.5bn on the NHS by 2023-24 and has raised concerns inside the Treasury. But Downing Street hopes it will demonstrate that the government still has a domestic agenda outside Brexit. Theresa May said the strategy would achieve “a significant part of our ambition to deliver true parity between physical and mental health”.

“We have already done a lot to end the stigma that people with mental health face and make sure they can reach out for help, and this expansion, part of our long-term plan for the NHS, will make a real difference and make the UK a world leader when it comes to both the money it spends on mental health services and the support and treatment it offers those who need it.”

Labour says that almost a sixth of under-16s in England who suffer a mental health crisis are forced to wait more than four hours in A&E. The number is three times higher than it was in 2014, according to data from a freedom of information request.

May said in June that a long-term plan would be drawn up, just after she announced a large spending increase to mark the 70th anniversary of the health service.

Simon Stevens, the chief executive of NHS England, was given an ambitious brief to use the money to equip the health service to cope with the demands of an ageing population as it suffers a staffing crisis across almost all specialties.

Stevens was told that the plan must show how waiting times could be put back on track in areas such as A&E, cancer care and planned operations; stop hospital trusts ending up in the red; solve under-staffing; deliver the long-promised integration of health and social care; improve preventive care; push through unprecedented improvements in mental health care; and embrace technology and spread best practice.

The Centre for Mental Health, a charity, said the measures would help “scale up the level of support offered to people with mental health difficulties”. It added: “A truly comprehensive set of access standards, similar to those for physical health services, could help to bring parity a step closer if it is backed up with enough funding and long-term investment in the workforce we need to offer people the right care at the right time.

“The government must now also step up and invest in public health, social care, early years and youth services if it is serious about achieving parity for mental health. Without it, too many people will continue to need NHS support and not get the help they need until they reach crisis point.”

Dave Munday, chair of the Mental Health Nurses Association, said: “Alongside waiting-time standards there must be safe staffing standards in mental health services. Whatever money is put into the NHS through the long-term plan, it must also be weighed against the millions that have been cut from parts of the NHS and all other public services.

“With the double-digit percentage cuts to local authorities, we’ve seen services that have supported people with mental ill health erased.”