Emergency doctors in the UK have issued a safety warning over water beads, which could be given to children as gifts over Christmas.

The brightly-coloured soft plastic beads, also known as jelly balls, sensory beads, or water crystals, are marketed as crafting tools and homeware items – but also toys.

They are typically only a few millimetres but some can expand up to 400 times their original size in about 36 hours when exposed to liquid, creating a hidden danger. If a child swallows one, they can cause bowel obstruction and the consequences can be fatal.

The Royal College of Emergency Medicine (RCEM) advises that the beads should be kept well away from children aged under five.

The “safety flash” is for doctors to make sure they know what to look out for while on duty, but the college also wants parents and carers of be aware of the dangers.

The beads are not visible on X-rays.

They can also cause choking and have harmed children in the UK, with reports linking them to deaths overseas, the government’s Office for Product Safety and Standards said in September.

It added the beads should only used by older children or vulnerable adults under close supervision.

The RCEM advises parents and carers to seek medical attention immediately if there is any suspicion a child may have swallowed a water bead.

“Having seen the effects first hand, which can be devastating and detrimental, we are asking people to please think twice about the dangers these pose while selecting and gifting presents this season,” Dr Salwa Malik, vice president of the RCEM, told PA Media.

The RCEM alert also highlights the potential danger to children and vulnerable adults of button or coin batteries and magnets, if swallowed.