Many women rely on the morning-after pill as a method of emergency contraception but new guidelines warn it can fail when taken by those who weigh 11 stone or more.

Those who have a body mass index (BMI) that is over 26 are also at risk according to new guidance from sexual health experts.

The news is likely to be alarming for many, given that more than half of UK women are thought to weigh more than 11 stone.

Levonelle and ellaOne are the two most popular morning-after pills available and both are thought to be less effective for heavier women.

It is understood the drug may be diluted in bigger women or broken down more quickly by their bodies.

The new advice from the Faculty of Sexual and Reproductive Healthcare (FSRH) urges pharmacists and doctors to inform fat women of the risk of failure.

They have been told to recommend two morning-after pills be taken - double the dosage - or to urge the use of the emergency coil instead.

Having a BMI over 26 or weighing over 11 stone has been linked to reduced effectiveness or the morning after pill

FSRH is a faculty of the Royal College of the Obstetricians and Gynaecologists.

Dr Jane Dickson, vice president of FSRH, said: 'The morning-after pill works by delaying interfering with the release of eggs and disrupting fertilisation through delivering a higher dose of the hormone progestin that is found in regular birth control pills.

'We believe there is evidence to suggest that in heavier women, the drug may be less effective because the drug is diluted in their blood stream.

'The 11 stone figure is based on research. It is something of an arbitrary figure and it may be 15 stone is the danger point for some women. But for safety 11 stone or a BMI over 26 is the level we can say weight may create a risk.'

Dr Dickson explained that larger women worried about pregnancy after contraception failure or unprotected sex could take two morning after tablets.

BMI has become a benchmark in measuring whether you're a healthy weight for your height.

FSRH is recommending health professionals advise women that an intrauterine device (IUD, or the coil) is the best emergency contraception.

Dr Dickson said: 'The coil's effectiveness is not affected by a woman's weight as it works differently to prevent fertilisation - it's toxic to sperm and eggs and works locally.

'And weight issues aside, it is more effective than the morning after pill.'

Trials suggest the failure rate for the IUD as emergency contraception is lower than one per cent.

However, research reveals that 95 per cent of women are issued the morning after pill when obtaining emergency contraception, probably due to the convenience of visiting a pharmacy. The IUD must be fitted by a health professional at a GP surgery or a sexual health service.

Over half of women in the UK are over the weight specified in the new guidelines. The coil's effectiveness has not been linked to weight or BMI

HOW HAVE THE RECOMMENDATIONS COME ABOUT? The link between weight and the morning after pill's effectiveness has been raised in previous studies but has proved controversial. Despite the reported link, previous guidance in the UK has not been clear on the subject until now. The issue first came to light in 2012, when an Edinburgh University study found that obese women were more than three times as likely as those of normal weight to become pregnant after taking the morning after pill. The link was particularly strong for levonorgestrel-based pills. Then in 2013, the European Medicines Agency (EMA) forced the makers of Levonelle (Norlevo in the US) to add a warning to its packaging that the product might be less effective for overweight women. But after a review, the agency had a u-turn in 2014 and said it is suitable for heavier women. Last July, a small study - with just 10 participants - found the amount of the hormone present in the 'overweight' women was 50 per cent less than in their lighter counterparts, meaning there was an increased risk of pregnancy. Advertisement

Dr Dickson explained that both types of morning after pill are thought to be less effective for heavier women but the link was particularly strong for levonorgestrel-based pills (such as Levonelle).

She said: 'ellaOne, which contains ulipristal acetate, could be less effective for overweight women but not to the extent of Levonelle.

'This is probably because ellaOne works better overall.'

Levonelle is thought to prevent up to 95 per cent of pregnancies if taken within 24 hours, and ellaOne is thought to remain up to 95 per cent effective throughout a five-day window.

Following the FSRH's warning, the manufacturers of leading emergency contraceptive ellaOne has been quick to defend its effectiveness.

HRA Pharma marketing manager Clare Newins said: 'We are keen to ensure that women do not misunderstand this new guidance and wrongly believe that there are no suitable oral emergency contraceptive options available to them.

'In fact, ellaOne continues to be the most effective oral option for most women at the standard dose (30mg single tablet) regardless of their weight or BMI.'

She pointed to research that shows over three quarters of women visit the pharmacy as their first port of call for emergency contraception where the IUD is not immediately available.