Doctors believe a quarter of requests for sick notes are suspicious.

GPs across Sussex receive up to 400,000 requests for certificates every year of which they estimate up to 100,000 are bogus.

But they insist the number of people on sick leave could be reduced if it was not for delays in treatment.

They blame employers for the number of spurious requests and said bosses could slash absenteeism by offering staff more flexible-working hours and support in the workplace.

Thousands of working hours are lost every year in Sussex through people "throwing sickies".

Workers who are ill are normally given a week's grace, known as self-certification, after which they must obtain a note from their doctor.

Dr Ann Robinson, one of the GPs who took part in the research for Norwich Union Healthcare, said: "GPs want to treat genuinely ill patients and don't want to act as policemen, identifying those who are claiming bogus sick notes.

"Employers need to be more flexible with their workforce and hospital services need to provide fast track diagnostic and treatment centres so people can get back to work as quickly as possible.

"It's well known that the longer you're off work, the harder it is to get back."

The latest figures come from a survey of GPs across the South who were asked how many sick-note requests they received every year and how many they considered valid.

The results reveal the average family doctor receives 457 requests a year.

There are 876 GPs serving a population of 1.5 million in Sussex, suggesting a total of up to 400,332 requests across the county.

Almost 25 per cent of requests for sick-notes doctors get yearly are suspect and more than one in ten is clearly bogus.

Almost a quarter of the GPs interviewed said up to 20 per cent of their patients were unable to work due to health reasons.

The survey, carried out by research firm Dr Foster, showed back pain was the most common excuse used, followed by depression, workplace stress and other stress-related problems, and flu.

Many of the GPs said they believed more than a third of their patients who were signed off sick could actually work a few hours a day, or in a slightly different role, but employers were not encouraging them to return to work.

They said better public education about the impact of a poor diet on health would be most effective in reducing sickness.

As part of the research, 1,000 workers were asked about their attitudes to taking time off.

More than seven per cent admitted they would consider asking their GP for a bogus sick note.

Twice as many men as women said they would cheat the system.

Circumstances in which workers said they would consider asking for a bogus sick note included a personal crisis they were too embarrassed to tell the boss about, difficulties at work, the refusal of a holiday request and fatigue.

Dr Doug Wright, clinical development manager at Norwich Union Healthcare, said: "One of the main themes of the report is how much time doctors devote to matters other than medical ones."

The British Medical Association (BMA) is so concerned it recently struck a deal with the Government to investigate whether other agencies could take responsibility for sick notes.

A spokeswoman said: "We are looking at whether companies could use their occupational health services to assess whether employees are fit for work."