Health experts are warning holidaymakers to check the safety of their hotel swimming pool before diving in, as they could risk catching a serious stomach bug.

Dozens of inspectors from Check Safety First visited hotels in nine countries to test the hygiene of their pools and found many hoteliers did not understand the dangers associated with dirty water or how to keep pools clean.

Neglected pools can quickly become a breeding ground for the cryptosporidium bug, a form of gastroenteritis which causes diarrhoea, vomiting, stomach cramps and fever that can last for up to six weeks. Because of the symptoms it is often misdiagnosed as food poisoning, so sufferers do not realise the pool is to blame.

Last summer tourists in 11 hotels in Mallorca and Menorca contracted the bug, with more than 100 people falling victim to it at the Hotel Alcudia Pins, Mallorca.

'There are no consistent rules to dictate the water quality in hotel pools and where local regulations do exist they are rarely monitored closely enough to sufficiently protect holidaymakers,' said Steve Tate, head of Check Safety First, which has been monitoring pool safety since 2000. Most of the hotels in Egypt, Greece, Mexico, Morocco, Portugal, Spain, Tunisia, Turkey and the Dominican Republic that invited Check Safety First inspectors were largely found to have unsatisfactory hygiene procedures, but they trained staff in how to keep them safe, and those with satisfactory hygiene levels are now listed on the website www.checksafetyfirst.com

If the pool is cloudy so the bottom can't be seen, if it has debris on the surface or the floor, is scummy with dirt or grease around the sides at water level, smelly, overcrowded with people or underfilled, a pool should not be used.

The organisation is a member of the Foreign and Commonwealth Office's 'Know Before You Go' campaign.