The number of newly diagnosed cases of gonorrhoea in England soared by 25% in 2011 amid fears that the sexual infection is becoming untreatable.

There have been cases in Japan and Europe of infections that resist first-choice therapies.

The Health Protection Agency said the threat of gonorrhoea was "very concerning".

It said there was a 2% overall rise in the number of new sexually transmitted infections diagnosed in 2011.

These figures must act as a wake-up call Lisa Power, Terrence Higgins Trust

Gonorrhoea is caused by bacteria and can result in infertility. It is treated by antibiotics, but some are failing.

The head of sexually transmitted infection surveillance at the HPA, Dr Gwenda Hughes, said: "We are very concerned. It is a global problem.

"[The gonorrhoea bacterium] is very successful at developing resistance to every treatment used in the past few decades.

"We are worried that in the next five years, or some point in the future, that this is going to be a very difficult infection to treat."

Even the most resistant forms of the disease are not yet untreatable.

Professor Cathy Ison, a gonorrhoea expert at the HPA, said that in the past, when resistance emerged, a new drug would be used.

However, she warned: "We don't have a new drug."

Cases rising again

Between 2009 and 2010, the number of new sexually transmitted infections being diagnosed fell for the first time in more than a decade.

However, cases have since increased, with nearly 427,000 new infections diagnosed in 2011.

More accurate tests and more people being tested explain some of the rise.

The message is clear: whatever your age, you should always use a condom Department of Health

But the HPA said it was concerned about "ongoing, unsafe sexual behaviour" in at-risk groups.

Dr Hughes said two groups - young adults and men who have sex with men - were of particular concern.

There was a 4% drop in the number of diagnoses of chlamydia in young adults. However, this has been attributed to fewer people getting screened.

Sexually active people under 25 are advised to be checked annually or when they have a change of sexual partner. Yet the number of tests fell from 2.3 million to 2.1 million between 2010 and 2011.

The government said too many people were not taking care of their sexual health.

A Department of Health spokesperson said: "Sexually transmitted infections can lead to infertility and other serious health problems. The message is clear: whatever your age, you should always use a condom."

Lisa Power, from sexual-health charity the Terrence Higgins Trust, said: "These figures must act as a wake-up call, not only to sexually active people but also to the government and public-health services.

"They represent a step backwards for the nation's sexual health. The emergence of drug-resistant strains of gonorrhoea is just one consequence of continued high rates."