Men are being targeted in a cruel sex hoax which looks like a letter from a clinic but links to a 60p-a-minute chatline.

Letters purporting to be from a hospital inform men they have been in sexual contact with a woman patient and they urgently need a check-up for sexually transmitted diseases.

A phone number for the clinic is included but the number is for an adult chatline.

Police fear the letters could cause trouble with wives and partners.

A spokesman said: "Men could be left with a lot of explaining to do. This is appears to be a very spiteful and sadistic prank."

Police and the Independent Committee for the Supervision of Telephone Information Services (ICSTIS), a Government watchdog which oversees high-tariff phone lines, are looking into the hoax.

One man in Woodingdean, Brighton, sent his letter to police.

Headed Royal Sussex County Hospital, it reads: "I am writing to inform you that one of our recent patients has given your name as a sexual partner of hers.

"Therefore we strongly recommend you contact us as soon as possible for a free confidential check-up.

"Please do not be alarmed at this letter as in most cases the results are either negative or can be treated with antibiotics if caught early enough.

"We have a walk-in clinic where appointments are not necessary.

"The remainder of the week you will only be seen by appointment.

"Please telephone the above number to make an appointment or to talk to one of our trained counsellors."

Police said the Brighton hospital had no knowledge of the letter.

Detective Sergeant Phil Waters said: "We want to hear from anyone who may have received such a letter.

"Our advice to men is to ignore the contents."

One theory is the letters were designed to get men to use the high-tariff telephone number with the author receiving a share of the 60p a minute cost.

But Rob Dwight, spokesman for ICSTIS, said it was a lot of work for little return and the operators of the adult chat line appeared to be innocent.