Cases of chlamydia and herpes have more than doubled in ten years, while syphilis – which was almost eradicated ten years ago – have risen fourfold.

Researchers say one in five adults receiving HIV treatment is at least 50 – twice as many in that age bracket as a decade ago, with a particular surge in the number of pensioners being diagnosed. They claim men who take Viagra are especially vulnerable to HIV.

The warnings come from doctors writing in the magazine Student BMJ, who say four out of five people aged 50-90 are sexually active.

Public health officials have grown increasingly concerned about the spread of sex infections among older people. Doctors blame increasing separations among the age group and the subsequent search for new relationships, often through internet dating agencies. But many fail to heed safe-sex messages to use condoms, believing they are aimed only at younger people.

Research has also suggested older ‘swingers’ are pushing up rates of sexually transmitted infections (STIs), with some of the highest rates among heterosexual couples who swap partners at organised parties and indulge in group sex.