Elaine Riding expected to be a mother within a year of undergoing IVF treatment. But at the age of 37, the sales manager is facing an increasingly hard battle to have children.

"We presumed, wrongly, that if you do it the right way - you get married, get a roof over your head and have a little money in the bank - everything would be OK," said Mrs Riding, who has medical problems that drastically reduce her chances of conceiving.

The Ridings, from Oxfordshire, are among thousands of couples who, experts say, are part of a new childless generation unable to conceive because they have left it too late.

The British Fertility Society (BFS) is now issuing a stark warning to women not to delay trying for children until their late thirties and forties in the false belief that they are guaranteed to be able to conceive through IVF.

Dr Richard Kennedy, from the BFS, says that women are lulled into a false sense of security by success stories of couples becoming parents late in life and their "very high expectations" of what fertility treatment can achieve.

"It's good they should pursue their careers, but Government and employers must take on board that women are more fertile in their younger years."

New figures from fertility clinics show that there has been a significant increase in the number of older female patients. For example, the number of women aged 40 and over seeking treatment at London's Bridge Clinic has risen from 174 in 2001 to 294 this year. Just under half of women attending the Lister Fertility Clinic in Chelsea are now aged 38 and above compared with a third of patients five years ago. The Newcastle Fertility Centre says that the average age of patients is now 35; five years ago it was 30.

Britain is facing a fertility time bomb that will have a huge social impact over the next decade. Some experts predict that the number of people unable to conceive naturally will double over the next decade, which means that as many as one in three couples will have trouble getting pregnant.

A woman's fertility starts to decline gradually from the age of 27, and after 35 her chances of getting pregnant will have halved. Yet the average age of British women at the time of giving birth has risen from 22 to 28 over the past 30 years.

But despite a massive increase in life expectancy, a woman's fertile years have only increased by one year. The average age for the start of the menopause has risen to 50.

Melanie Davies, a consultant obstetrician at University College London and one of the authors of the report, said that her clinic is full of women who have achieved everything in their lives, but left it too late to have children. But she said it was wrong to blame women for the crisis.

"Women are the victims because society demands so much of us, and for us to succeed in every aspect of our lives," she said. "The problem, though, is that despite fantastic advances in science, there is always going to be a cut-off point where your eggs will no longer be viable."