Mental health spending needs to double by the end of the next decade to put it on par with physical illnesses, new analysis suggests.
Ministers and health leaders have pledged that mental health care will be given "parity of esteem" to physical illnesses.
Now research by the Institute for Public Policy Research (IPPR) think-tank found that, in order to achieve this, NHS spending on mental health in England must double by 2030.
Spending needs to rise to £23.9 billion a year by 2030, from its current level of £12 billion, the IPPR said.
The analysis comes as 15 organisations including medical bodies, charities and organisations representing mental health trusts wrote to Prime Minister Theresa May to implore her to turn her commitments on mental health into "ambitious and bold action".
The letter, led by the charity Rethink Mental Illness, states: "We hear far too often from people experiencing mental health problems who are having to wait months, sometimes years, for treatment.
"This, alongside a lack of sufficient prevention and social care services, mean too many people are reaching crisis point, resulting in lifelong harm to their quality of life, not to mention impacting on NHS and wider public finances.
"In light of the welcome extra funding for the NHS, now is the time to translate your commitments into ambitious and bold action."
The IPPR report goes on to describe how additional funds should be spent including early intervention; scaling up treatments for common illnesses such as depression and anxiety; better community care for those who suffer severe mental illness; and improved crisis care.
Harry Quilter Pinner, senior research fellow at IPPR and lead author of the report, said: "Parity of esteem, to which the NHS has committed, cannot just be talking about mental health as well as physical health. It has to lead to equal access to treatment and better outcomes for patients.
"Only a minority of people with mental health conditions currently receive access to treatment. We would not tolerate that for cancer, diabetes, heart disease or any other illness, and we should not tolerate it for mental ill-health either.
"Once that is understood, the scale of extra funding needed becomes clear."
Brian Dow, deputy chief executive at Rethink Mental Illness, said: "The Government has committed to achieving parity between mental and physical health - and that of course does not come for free.
"Because of this report, we now know what that cost is for the first time. That's why, today, we, together with 14 other organisations, are calling on the Prime Minister to deliver on her promise of parity so that everyone who needs it gets high-quality mental health care to help them live healthy and fulfilling lives."
A Department of Health and Social Care spokeswoman said: "Parity between mental and physical health is a key priority for this government, which is why we are transforming services with record amounts of funding and have introduced the first ever waiting time standards for mental health.
"But we want to go further, which is why the NHS long-term plan, backed by an extra £20.5 billion a year investment by 2023/24, will outline our next steps towards parity of esteem, so more patients are able to access mental health services."
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