Britain has become a "Prozac nation", with the use of antidepressants spiralling out of control amid a crisis in mental health care, the Liberal Democrat leader, Nick Clegg, will warn today.

In a speech to the Guardian public services summit, Clegg will commit his party to a maximum 13-week wait for NHS treatment for mental health problems. If the NHS misses the target, the patient will be entitled to go private and make the health service pick up the bill, he will say.

The government is committed to an 18-week deadline for consultant-led care only, from March.

Clegg will highlight figures showing one in four Britons suffering from mental health problems at some point in their lives, with one in six at any one time. The cost of mental ill-health is estimated at £77bn a year.

He will also point to an "explosion" in antidepressant use, with 31m prescriptions issued in 2006, including 631,000 for children.

"Britain has become the true Prozac nation. I believe this trend has gone too far," Clegg will say. "That is not to say that medication has no role to play in tackling mental health problems; of course it does. But [it] should not be the default option, prescribed by doctors because of a lack of access to psychological therapies ... pills must not be a crutch for the wider issues in our society which cause mental health problems."

The Lib Dem leader will accuse politicians of "shamefully" turning a blind eye to the scale of mental health problems. He argues that while the government has invested a lot more in the NHS, mental health has lost out to the demands of bureaucracy and reform, with money channelled into acute services, benefiting only 2% of users.

He will welcome the £170m announced by the government in the autumn for psychological therapies but call for new targets for community treatments, where 98% of patients are seen. There is, he will argue, "a lottery in mental health service waiting times".

Clegg will say: "In Leicestershire, the longest waiting time for cognitive behaviour therapy is almost a year and a half. In Gloucestershire you can wait almost two years for eating disorder treatment. And in Plymouth, patients can wait for over three and half years for a psychotherapy assessment. This is a heartless, brutal way to treat some of the most vulnerable people in our society."

The speech is one of two Clegg is making today. The other, on his economic agenda, will reaffirm his commitment to financial discipline and no overall tax increases. Clegg has appointed Jeremy Browne, number two in the party's Treasury team, to identify 3% of cuts in every department, to produce annual savings of £20bn across government for Lib Dem priorities.