Waiting time targets for people with mental health problems in England are to be introduced for the first time, Nick Clegg has announced.
People with depression should begin “talking therapy” treatments within 18 weeks, from April.
Young people with psychosis for the first time will be seen within 14 days – the same target as cancer patients.
The deputy prime minister unveiled the move ahead of his speech to the Lib Dem conference in Glasgow.
BBC assistant political editor Norman Smith said this would be the “big announcement” of Mr Clegg’s conference address, with the rest of it focusing on what aides called “vision and values” rather than detail.
Under the plan, suicidal patients get the same priority as those with suspected heart attacks.
Mr Clegg will tell Lib Dem activists the £120m plan is the first step in reforming “Cinderella” mental health services.
The money to pay for it will be reallocated from other parts of the health budget – but it is a coalition policy not just a Lib Dem manifesto pledge.
Mr Clegg will also pledge to extend the extra money for mental health in next Parliament if the Lib Dems are in government, to introduce similar targets for conditions such as bipolar disorder and eating disorders.
Lib Dem Health Minister Norman Lamb said it was a “watershed” moment that achieved “genuine equality between mental health and physical health”.
He told BBC Radio 4’s Today programme: “This is a simple fact of discrimination. If you have cancer, you get access to a specialist within two weeks. If you have a first episode of psychosis, it’s completely haphazard and that is outrageous. There’s a moral and an economic case to do this.”
Half of the £1bn Mr Clegg announced for the NHS at the start of his party conference conference would be spent this way.
The Lib Dem leader is attempting to set out a distinctive message on the NHS ahead of May’s general election and will make his mental health reforms a “red line” issue in any coalition negotiations.
Aides said Mr Clegg has not suffered from any mental illness but has campaigned on improving treatment for patients his “whole political career”, making his first speech in the House of Commons on the issue.
He will say: “If you are having a breakdown, if you are thinking of harming yourself, for any emergency which takes you to A&E, you’ll get the help you need – just as if you had gone to hospital with chest pains or following an accident.
“These are big, big changes. And in government again the Liberal Democrats will commit to completing this overhaul of our mental health services – ending the discrimination against mental health for good.
“And while I know not everyone in the party is going to agree, I can tell you now: I want this smack bang on the front page of our next manifesto. One of a small number of top priorities. This is a great liberal cause. Let’s be the first political party to give mental health the status it deserves.”
Mental health problems are estimated to cost the economy around £100bn a year and around 70 million working days are also lost annually.
The Lib Dem leader will say much progress has been made in ending the stigma around mental health but more needs to be done, adding: “I want this to be a country where a young dad chatting at the school gates will feel as comfortable discussing anxiety, stress, depression as the mum who’s explaining how she sprained her ankle.”
The announcement was welcomed by mental health charities.
Mark Winstanley, chief executive officer at Rethink Mental Illness, said it had “the potential to improve the lives of millions”, while Centre for Mental Health chief executive Sean Duggan said it would “help to overcome the current postcode lottery” accessing essential services.
Sue Baker, from the Time to Change charity, which campaigns to end the stigma around mental health, said: “I know money is tight across the NHS, but there should not be any level of discrimination where one health issue is not as equally funded as other areas.”
Labour welcomed the announcement but accused Mr Clegg allowing mental health services to “fall into crisis” through budget cuts.
Shadow health secretary Andy Burnham said: “For over four years Nick Clegg has let mental health services slip backwards. Waiting times for talking therapies have got longer and people are struggling to get the support they need.”