Gay and bisexual men in London are being warned about growing rates of an “aggressive” strain of chlamydia.
LGV (lymphogranuloma venereum) arrived in the UK in 2004, health experts say.
New figures from the Health Protection Agency show that cases in London increased from 135 in 2009 to 350 in 2010, the highest number to date.
Infections have also been seen in Manchester and Brighton.
According to sexual health charity Terrence Higgins Trust, symptoms can include bleeding and discharge from the rectum, painful inflammation and painful anal abscesses or ulcers.
It can cause permanent damage if untreated. Most men who pick up the infection show symptoms within a few weeks.
Ben Tunstall, head of health improvement at Terrence Higgins Trust, said LGV bacteria are passed on through unprotected anal sex, fisting or on sex toys shared between partners.
He said gay men should use condoms and latex gloves for sexual activity and should not share sex toys. Men with symptoms of infection should go to a sexual health clinic, as GPs may misdiagnose the problem.
LGV can be cured with a three-week course of antibiotics.
Mr Tunstall said: “This is an alarming increase. LGV is a particularly nasty infection, which also increases your risk of picking up or passing on other STIs, including HIV, Hep C, and syphilis. It can be easily treated, but it’s obviously far better for you – and of course your partner – not to get it in the first place. The best way to guard against LGV is to use condoms during sex.”