Thousands of women in the UK may be taking antidepressants prescribed by their GPs without knowing that the pills, which are hard to stop taking, could cause birth defects in unborn children.

The problems relate to a class of drug known as SSRIs (selective serotonin reuptake inhibitors), which includes Prozac and, in particular, the British-made Seroxat.

Several studies have shown a link to birth defects, particularly malformed hearts, in a small proportion of the babies born to women who were taking the drug in the early weeks of pregnancy.

Most GPs in the UK believe that these drugs are safer than older antidepressants. Seroxat has been marketed to women as a drug to relieve anxiety and depression.

In the US, the Food and Drug Administration, which licences medicines, issued a warning in 2005 and changed the status of Seroxat, which is sold there under the brand name Paxil. The FDA warns doctors "not to prescribe Paxil in women who are in the first three months of pregnancy or are planning pregnancy, unless other treatment options are not appropriate".

The UK regulator, the Medicines and Healthcare products Regulatory Authority (MHRA) wrote to doctors telling them to prescribe Seroxat for pregnant women only when the benefits outweigh the risks. The National Institute for Health and Clinical Excellence (Nice) also urged caution.

But experts including Dr Tim Kendall, joint director of the National Collaborating Centre for Mental Health which wrote the Nice guidelines on depression, say GPs are still giving SSRIs to pregnant women and other women of childbearing age without warning of the potential dangers.

Most psychiatrists he knew would no longer prescribe Seroxat, said Dr Kendall. "But in primary care it is still quite widely prescribed. GPs are quite flooded with advice. It is unlikely they will have picked up a specialist piece of advice from Nice about mental health."

In the US, a series of legal actions is about to begin. Lawyers representing women suing the manufacturer of Seroxat, GlaxoSmithKline, say the British company knew or should have known about the birth defects more than 10 years ago. GSK denies it, saying it told the authorities as soon as it was aware of the issue.

All the SSRIs are implicated. According to David Healy, professor of psychiatry in Bangor, Wales, who has been asked to give evidence in the US cases, the rate of birth defects is doubled from 2% in the general population to 4% of those on the drugs. The rate of major defects rises from 1% to 2%. The general rate of miscarriages is 8%, but 16% of women on Seroxat miscarry.

Concern about depression in pregnancy has grown in recent years. Midwives at antenatal clinics are increasingly encouraged to ask pregnant women about their mood and feelings, to pick up any signs of depression which could cause them to harm themselves or fail to bond with the baby when it is born. Talking therapies should be an option but are often in short supply.

Kendall believes the issue of the SSRIs in pregnancy needs to be addressed. "They are addictive," he said. "The question is should we warn young people before they take them that if they think they might be wanting to get pregnant, these drugs are quite hard to get off?"

"From the late 1980s there was work which very clearly showed that the SSRI group of drugs ought to be regarded as posing a high risk of birth defects to women in the early stages of pregnancy or when they didn't even know they were pregnant," said Healy.

One of the SSRI manufacturers, Pfizer which made Zoloft, indicated in the prescribers' bible, the British National Formulary, that their drug was not recommended for women who might become pregnant. This warning, said Healy, "probably did very little to deter women from taking the drug or doctors from prescribing it, but GSK went further and actually promoted the drug to women of childbearing age".

All SSRIs pose a risk, said Healy. The danger with recent warnings from the FDA and MHRA about Seroxat is that GPs will switch women to another drug of the same class.

Emily Jackson, professor of law at the London School of Economics, believes there may be a case for legal action in the UK. Cases could potentially be brought against either the GP or the manufacturer for a failure to warn of potential risks.

"The group of patients who often receive an inadequate warning are women who are not currently pregnant or trying to become so, but who are not warned that there is a danger that they will become addicted to paroxetine [Seroxat] while they are not pregnant, and will find themselves unable to stop taking it once they become pregnant, perhaps many years later," she said.

She raises the possibility of an action under the Congenital Disabilities Act of 1976 on behalf of a child injured by antidepressants prescribed for the mother.

She compares the "no alcohol" message put out by the department of health to pregnant women to the "more equivocal advice" on Seroxat and pregnancy. In both cases, she says, "it seems that women are not to be trusted with making choices for themselves".

GlaxoSmithKline argues there is still insufficient scientific evidence to prove that the drugs directly cause defects.

A spokesman said: "Tragically, birth defects can occur whether or not the mother was taking medication during pregnancy. We have monitored reports of foetal exposure to paroxetine since the first studies of the drug and there was no indication of increased risk from studies, adverse event reports or any other source until the summer of 2005.

"As soon as we became aware of a potential increased risk, we promptly notified regulatory authorities and physicians. We strongly believe that doctors should be advised of the potential risk of medicines before prescribing them, and the potential risk of paroxetine use during pregnancy is detailed in the information provided with the medicine."