A report from fertility specialists at the Royal College of Obstetricians and Gynaecologists, commissioned by the government, is thought to recommend the controversial age limit.

Guidance from the Department of Health to health authorities, some of which currently refuse to fund IVF, is expected early next year.

There is more demand than ever for IVF

The formula would end the "postcode lottery" of IVF treatment in Britain in which some health authorities provide the treatments free while others do not.

Junior Health Minister John Hutton has already pledged that the government would move towards "fairer access" to NHS infertility services - but only for those "most able to benefit" from them.

But the prospect of restricting NHS treatment to the under-35s is likely to provoke a furious response from fertility support groups.

The chances of a successful pregnancy using standard IVF do diminish swiftly as a woman approaches the age of 40, although many women have had children much later following fertility treatment.

And scientists are working on new techniques which could increase the success rate for older women, and cut the number of babies born with congenital abnormalities such as Down's syndrome.

Huge cost for couples

If carried out privately, IVF can cost anywhere from several hundred to several thousand pounds to achieve pregnancy, depending on at which hospital the treatment is carried out and the number of attempts needed.

Newspaper reports also suggest that the number of NHS-funded attempts will be restricted to two per woman, and that treatment could be refused to some women who are overweight or smoke - both factors which reduce the chances of success.

Report expected later this year

A spokesman for the Department of Health said that the report was expected later this year, with fresh guidance to health authorities issued shortly afterwards.

IVF involves treating the woman with fertility drugs, which increase the number of eggs that can be "harvested" from her ovaries with a simple operation.

A small number of these are then fertilised with the partner's sperm outside the body, then implanted in the womb, where is is hoped that one or more will take hold and develop.

Multiple births are commonplace following successful IVF treatment.