Depression and anxiety are now the most common reasons for people starting to claim long-term sickness benefits, researchers in London have said.
Some people fear going back to work will trigger a relapse
They have overtaken musculoskeletal conditions such as back pain, which used to prompt most such claims, the team told the British Medical Journal.
They called for more to be done to help people with depression return to work.
The King's College London team said these benefits - available after six months' sickness - cost £13bn a year.
They add an estimated 176 million working days were lost in 2003, up 10 million on the previous year.
Data from the Department of Work and Pensions suggests around 35% of people claiming Incapacity Benefit in 2002 had mental or behavioural disorders, compared to 22% with musculoskeletal conditions.
The researchers' call for action comes as the House of Lords backed down in a confrontation with MPs over extending disability discrimination legislation to people who suffer bouts of depression.
Long-term sickness is counted as anything over six months - when people become eligible to claim benefits such as Incapacity Benefit.
The government recently announced that benefit, paid to 2.7 million sick and disabled people, was to be overhauled to remove disincentives to return to work.
Illnesses such as depression are treated using medication, although patients often say they would prefer therapies such as psychotherapy or counselling.
However, there are long waits for "talking therapies".
The King's researchers also say there are too few occupational therapists in the UK - just one specialist for every 43,000 workers.
They say occupational physicians can help find ways for people to return to work after a long illness.
Led by psychiatrist Max Henderson, the researchers write: "Both employers and patients require a speedier response than is currently delivered, as the longer an individual remains off work, the more difficult a return becomes."
They highlight a successful project in Holland, where work-based psychological therapy improved health and reduced absences.
"If the government is serious about tackling the consequences of common mental disorders then innovative policies... will be required alongside research into the most effective and cost effective methods of delivering service," the researchers add.
"This would be a wise investment given the substantial economic and social costs engendered by the current service framework."
A spokesman for the Depression Alliance said employers were often not equipped to recognise the warning signs of stress and mild depression in workers, and so were unable to help them early on in their illness.
He added: "Talking therapies and one-to-one support are key to helping someone adjust to a working environment after a period of sickness when self-esteem is probably at an all time low.
"Employers also need support to understand the situation and their responsibilities. Unfortunately the necessary resources are scarce.
"And the incapacity benefit system itself is simply not designed to deal with the special requirements of people affected by depression.
"Service users often tell us that they end up in a vicious circle where they are unable to return to work or are forced back to work too quickly."
Kate Groucutt, a Policy Adviser with the Confederation of British Industries, said: "Nearly two thirds of employers have formal rehabilitation arrangements to assist employees returning to work, including flexible working, counselling, medical treatment and training courses.
"And over three-quarters of employers also have arrangements to help employees suffering from stress, including job reorganisation and access to occupational health services, which could help prevent more serious problems."