A sexually transmitted ‘superbug’ that is on the verge of becoming untreatable is sweeping across Britain, health experts warn.

Cases of the drug-resistant gonorrhoea strain have been confirmed in West Yorkshire, the West Midlands, London and the North East.

But there are likely to be many others which have gone unreported, officials fear.

It is caused by a bacteria quickly becoming immune to one of the last two available antibiotics.

Experts fear it will soon develop a resistance to the second drug – and there are no others in reserve.

Health officials are urging the public to limit casual sex and wear condoms with a new partner.

A highly drug-resistant type of 'super-gonorrhoea' is spreading across the country, with senior medics warning it may become untreatable. Neisseria gonorrhoeae (above), the bacterium responsible for the sexually transmitted infection gonorrhea

GPs have also been warned to be extra vigilant and ensure they prescribe the proper treatment.

But the spread of this ‘super’ sexually transmitted disease is further evidence of the growing threat of antibiotic-resistant bugs. For decades, antibiotics have been so overused by GPs and hospital staff that the bacteria have evolved to become resistant.

Doctors report that medicines including penicillin no longer work on sore throats, skin infections and more seriously, pneumonia.

Chief Medical Officer Dame Sally Davies has said the threat is as severe as terrorism – with patients dying from minor cuts after succumbing to drug-resistant bugs.

And only last week Chancellor George Osborne said that antibiotic-resistant bacteria will claim ten million deaths a year worldwide by 2050 – even more than cancer.

Public Health England yesterday issued an alert stating that cases of this super-gonorrhoea were continuing to rise and represented a ‘very real threat’.

So far 34 adults have been diagnosed with the strain since November 2014 but this is almost certainly only the tip of the iceberg. More than half of women and one in ten men never see symptoms so may pass on the infection unaware.

But they are at risk of serious complications and untreated, the disease can cause infertility or inflammation of the womb. It is particularly dangerous for pregnant women and may lead to miscarriage, premature labour or sight problems in the baby.

Usually gonorrhoea is treated with a jab, known as ceftriaxone, then by a pill called azithromycin.

But this strain is already partly resistant to the latter and experts are worried it will soon also become immune to the former. Dr Gwenda Hughes, of PHE, the Government body in charge of preventing the spread of bugs, said: ‘We cannot afford to be complacent.

The strain is 'highly resistant' to the antibiotic azithromycin, meaning medics are relying on a second drug, ceftriaxone, to treat it. Above an image of gonorrhea

‘If strains of gonorrhoea emerge that are resistant to both azithromycin and ceftriaxone, treatment options would be limited as there is no new antibiotic available.’

Dr Elizabeth Carlin, president of the British Association of Sexual Health and HIV, said: ‘The fact that we have resistance developing to one of the drugs we use means that we could potentially be left with only one drug to use. If it becomes resistant to that, we would be in a very difficult position.’

Doctors are also worried as the strain is now being reported in gay men so may start spreading even more quickly.

Dr Peter Greenhouse, a consultant in sexual health based in Bristol, said: ‘The problem is [they] tend to spread infections a lot faster simply as they change partners more quickly.’

In December, Dame Sally Davies wrote to GPs and pharmacists in December warning them that ‘gonorrhoea was at risk of becoming an untreatable disease’.

She reminded them to ensure they prescribed both the ceftriaxone jab and the azithromycin pill – some were omitting the injections.

Gonorrhoea is the second most common STD after chlamydia and is spread through unprotected sex. Annual cases have risen by a fifth, and some experts link this to a rise in casual sex.

There were 34,958 confirmed infections in England during 2014, most commonly in the under-25s, up from 29,419 the previous year.

Officials at PHE have tried to contact all sexual partners of anyone diagnosed with the superbug but have only managed to find 22 of the 50 partners.

More than 90 per cent of those they did track down were confirmed as having the infection.

This means there is a high chance the other adults also had it and may have passed it on.

If untreated, gonorrhoea can result in severe complications and lead to infertility or septicaemia in rare cases