Some boys are thought to have had unprotected sex with the woman, who is in her early 20s. Some may also have shared drug-taking equipment.

Health officials have sent a letter to parents of 700 children between the ages of 14 and 16 who attend Paignton Community College in Torbay, Devon, warning them of the dangers of the liver disease.

Doctors are also contacting all the woman's recent sexual partners.

Pupils who have come into contact with the woman, a known drug user, are being asked to take a blood test to see if they have contracted the virus are being offered immunisation. A helpline has also been set up.

Most recover

The college principal Trevor Yates said: "We do not know there is a proven definite link but there is concern that there could be.

"It must be emphasised that this young adult has not been hanging around the college. But we are part of the local community and something may have happened."

Hepatitis B is carried in blood and body fluids. It can leave sufferers with long-term liver damage, although most people make a full recovery and are left with a life-long immunity to the virus.

The disease is usually transmitted through unprotected sex, needle sharing or using unsterilised tattooing or body piercing equipment.

It can also be transmitted from a pregnant mother to her child. Health officials also fear that pupils shared drug-taking equipment with the infected woman.

'High-risk contact'

Symptoms of Hepatitis B, which include a mild flu-like illness and jaundice, can take up to six months to appear.

A spokesman for South and West Devon Health Authority said: "A case of Hepatitis B has recently been diagnosed in a young adult in Paignton. This is not that unusual but the case has generated some concern.

"Because of this a letter has been sent to parents of children at Paignton Community College, giving some background information.

"A number of individuals who may have had high-risk contact with the recent case in Paignton are being contacted and advised appropriately."