Workers who are off sick for longer than seven days will from now on receive so-called fit notes instead of sick notes from their GPs.
Doctors will still be able to say someone is not fit for work, but they will also be able to spell out aspects of jobs workers can still perform.
The onus will be on employers to help staff return to work in some capacity.
But the British Medical Association says more needs to be done so firms understand their responsibilities.
The overhaul of the sick note system was announced a year ago by government adviser Dame Carol Black, the national director for health and work.
She calculated that ill-health was costing the economy £100bn a year.
'Fit for work'
The Statement of Fitness for Work, or 'fit note', is a new medical statement that doctors will now be issuing. It is relevant to all employers in England, Northern Ireland, Scotland and Wales.
With the fit note the doctor will be able to advise their patient if they are "not fit for work" or a new option - "may be fit for work taking account of the following advice".
Patients will then be encouraged to discuss this advice with their employer to see if they can return to work.
Heather Matheson, human resources consultant: "There may be some conflict"
Dr Laurence Buckman, chairman of the BMA's GPs committee, said: "The responsibility will be on employers to act.
"If a GP decides their patient is capable of some form of work, for example if they've got back pain and they should temporarily avoid elements of their normal job, then it will be down to the employer to be flexible enough to accommodate them.
"However, we think much more needs to be done to ensure employers, in particular local line managers, have enough information about the changes."
The launch of fit notes is going to be revolutionary
Professor Steve Field, RCGP
And he said GPs would have to ensure they were not "drawn in to making comments they are not qualified to make" because they would be unaware of the details of a patient's working conditions.
Dr Buckman added: "Employers have a responsibility to provide adequate occupational health services and the government must encourage them to provide that if the overall plan to help more people back to work is to be truly effective.
"But, unfortunately, only one worker in eight has access to an occupational health doctor."
A thousand GPs have been trained by the Royal College of General Practitioners (RCGP) in how to complete the new forms.
OCCUPATIONAL HEALTH FACTS
172 million working days lost through sickness in 2007
This cost more than £100bn in lost productivity
2.6 million people were on incapacity benefits in May
Absences of four days or more account for 40% of days lost
Nearly six million people in the UK say they have a long-standing health condition
Sources: CBI; Black report
Professor Steve Field, of the RCGP, said: "There has been a lot of preparation for GPs, for employers, and employees but inevitably there will be some that are not up to speed.
"The launch of fit notes is going to be revolutionary because it will change the whole culture and we know that keeping people in work helps their physical and mental wellbeing."
Katja Hall, director of employment policy at the Confederation of British Industry, said: "This is a change that employers will welcome.
"All too often a person is signed off sick when they are able to manage some forms of their work."
However, unions have warned that handing someone a list of tasks they could perform while ill would not help them get back to full fitness.
Some GPs have been frustrated with the current system because they are unable to assess what work an employee can do if they do not know what their workplace responsibilities are.
Under the current system, a GP has the power to sign someone off work for six months before the case is passed onto a benefits administrator.
About 350,000 people a year transfer from sick notes to benefits, a figure which experts believe could be cut significantly with earlier and more effective intervention.
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