The Government is to inject an additional £2 billion a year into providing crisis services In England for people suffering mental health problems, Philip Hammond will announce.
The Chancellor will use the Budget on Monday to set out the first stage of the NHS's long-term plan to achieve "parity of care" between physical and mental health.
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The extra cash will help pay for the provision of "comprehensive" mental health support in every major NHS A&E department, ensuring anyone experiencing a mental health crisis can get rapid specialist help.
Officials say it will be backed up with more mental health ambulances and the establishment of dedicated mental health teams in schools, linking them to other support services.
'Too many people are already waiting'
Callers to NHS 111 will be directed to support services 24 hours a day and there will be greater access to services in the community, such as "crisis cafes" where people can get help without going to A&E.
The additional funding forms part of the extra £20 billion-a-year by 2023 for the NHS in England which Theresa May announced in June.
Shadow health minister Barbara Keeley expressed scepticism about the promise of extra funding.
"If this announcement is simply money that's already been promised, it will do little to relieve the severe pressures on mental health services that have built up because of this Tory Government's relentless underfunding of the NHS," she said.
"People with mental health conditions cannot afford to wait five years for meaningful action from this Government: too many are already waiting many months to access the treatment."
Universal Credit fears
The announcement was welcomed by Paul Farmer, chief executive of the mental health charity Mind, who said services had been underfunded for decades.
"This commitment ahead of the long-term plan indicates that mental health is rightly high on the Government's agenda, and has the potential to improve access to care, once detailed plans are clear," he said.
However he expressed concern the Government had yet to provide assurances that no claimants receiving benefits because their mental health made it difficult to work would lose income in the transition to Universal Credit.
"This has not yet been forthcoming," he said. "If the Government is really intent on prioritising the nation's mental health, it needs to guarantee nobody with mental health problems will be left without their income as a result of moving to Universal Credit."