More than 300,000 homes in Lancashire have been told that they may have to boil their drinking water after a microbial parasite was discovered in their supply.

United Utilities, which provides water and sewage services to around seven million people in North West England, found traces of the parasite cryptosporidium at Franklaw water treatment works near Preston, during routine tests.

Cryptosporidium can cause gastrointestinal illness with diarrhoea in humans. The parasite can cause acute, short-term infections, but symptoms can become severe in children and people with low immune systems.

The alert was initially issued by United Utilities last Thursday but the company have advised customers in Blackpool, Chorley, Fylde, Preston, South Ribble and Wyre to continue carrying out precautions till at least Wednesday as “low” levels of the parasite still remain in the supply.