Image caption If swallowed, magnets can trap parts of the intestine and lead to perforation

Young people are being warned about the dangers of magnetic tongue studs, after reports of serious medical emergencies.

The head teacher of a school in Manchester has sent out an alert to parents and spoken to pupils.

The studs give the wearer the look of a tongue piercing but without a hole.

If accidentally swallowed, the magnets - designed to be worn either side of the tongue - can stick together, trapping parts of the intestine and leading to perforation of the bowel.

While magnetic facial studs can be bought online, schoolchildren are thought to be fashioning their own tongue versions using magnetic earrings and ball bearings from toy puzzles.

In his letter to parents, head teacher Kevin Hogan from St Matthew's Roman Catholic High School said: "We have spoken to all pupils about this matter and warned them of the potentially harmful consequences of swallowing these magnets.

"If your child has accidentally swallowed one or more of these balls you should seek medical advice immediately."

Dr Anil Thomas George and Dr Sandeep Motiwale, of Queen's Medical Centre in Nottingham, wrote a letter to medical journal the Lancet, asking doctors to advise parents of the dangers.

They said two young children in the East Midlands had been admitted to hospital for surgery to remove magnets that had been swallowed.

They wrote: "Parents need to be alerted to the potential risk of silent bowel perforation and fistulation from accidental ingestion of magnets in children.

"Accidental ingestion of magnetic foreign bodies, which was once rare, has become more common owing to the increasing availability of toys with magnetic elements.

"A solitary ingested magnet can pass through the gut spontaneously. However, ingestion of multiple magnets or a single magnet along with another metallic part can cause them to stick to each other with forces of up to 1,300G [gauss], compressing the intervening bowel and leading to subsequent fistulation and perforation."