One million women who take Britain's most popular contraceptive pills are being warned they risk developing potentially life-threatening blood clots.
GPs have been ordered to inform patients that they are almost twice as likely to develop blood clots if they take some of the most popular birth-control tablets, including Yasmin, Femodene and Marvelon, compared with older products.
The alert came after scientists found that the so-called third generation contraceptives caused 14 deaths a year in France.
Britain's drug watchdog, the Medicines and Healthcare Products Regulatory Agency (MHRA) has ordered UK doctors to pay 'careful consideration' to individuals' risk factors before prescribing the combined hormonal contraceptives.
A history of deep vein thrombosis or very high blood pressure are among the conditions that would prevent a doctor recommending the drugs.
This comes after a review by the European Medicines Agency which found that the packaging of the pills should be updated to ensure that women are made aware of the risks of blood clots.
The MHRA wrote a letter to doctors on January 22 which stated that 'there is no need for anyone who has been using a combined hormonal contraceptive without any problems to stop taking it on the basis of this review'.
It added: "Combined hormonal contraceptives are highly effective in preventing unwanted pregnancy. They offer substantial benefits and these far outweigh the small risk of serious side effects."
Dr Sarah Branch, deputy director of the MHRA's vigilance and risk management of medicines division, said: "Women should continue to take their contraceptive pill. These are very safe, highly effective medicines for preventing unintended pregnancy and the benefits associated with their use far outweigh the risk of blood clots in veins or arteries.
"No important new evidence has emerged – this review simply confirms what we already know, that the risk of blood clots with all combined hormonal contraceptives is small.
"If women have questions, they should discuss them with their GP or contraceptive provider at their next routine appointment but should keep taking their contraceptive until they have done so."
The watchdog has written to the country's 60,000 GPs telling them they must, for the first time, go through a patient checklist when prescribing the pills, according to the Mail on Sunday.
The aim is to prevent the pills being given to women with a higher inherent risk of developing blood clots.
The newer, third generation, of contraceptive pills are prescribed about two million times a year and have proved popular because they are less likely to cause side-effects such as weight gain, headaches, breast tenderness and hair growth.
DVTs, which typically form in the leg, can travel up blood vessels to cause a potentially fatal blockage in the lung, known as a pulmonary embolism. They can also travel to the brain, triggering a stroke. Risk factors include being older and overweight, smoking or being prone to a type of migraine.
Pharmaceutical giant Bayer, which makes Yasmin and Femodene, said it would be making blood clot warnings more prominent on its packets in the wake of the warning.