Anglian Water has confirmed the animal has been linked to the source of cryptosporidium at Pitsford Water Treatment Works.

It got into the treatment process through a remote ancillary tank shortly before the parasite was detected.

Tests are now being carried out to understand the type of cryptosporidium found at Pitsford; the first time it has been found at the treatment works since it was built 50 years ago.

Peter Simpson, Anglian Water's chief operating officer, said: "Our investigations have concluded that this occurrence was due to a combination of unusual circumstances. We have already taken steps to ensure that this cannot happen again.

"We continually monitor the quality of our water. This monitoring enabled us to detect low levels of cryptosporidium at a very early stage, isolate the affected part of the process and take immediate steps to alert customers."

There have been 13 confirmed cases of cryptosporidiosis of the type of cryptosporidium in the water. A further 13 cases are still under investigation, the Health Protection Agency said.

More than 700 people reported symptoms of the illness to their doctor.

Anglian Water said water at Pitsford had been free from cryptosporidium since the early hours of Thursday, June 26, less than 24 hours after people living in Northamptonshire and Daventry were first told to boil their water.

The boil notice was issued after low levels of the parasite were found through routine monitoring.

Anglian Water said it had always taken the precaution of monitoring for cryptosporidium at Pitsford and similar sites and would continue to do so.

While the boil notice was in place, 1,000 miles of pipes were flushed out, some 2,500 customers visited mobile support units for advice and 3,000 customers were helped through the WaterCare Register, with more than 500 staff involved in the operation.

The boil notice was lifted on Friday, July 4, two weeks before Anglian Water had originally anticipated the service would be back to normal.