Householders in Gwynedd and Anglesey were issued with the boil water notice last week in what Welsh Water said was a "precautionary measure".

Anyone who became ill was also asked to contact their GP.

Welsh Water said the advice was likely to remain in place for at least two weeks.

The NPHSW said environmental health officers in Gwynedd and Anglesey are conducting detailed investigations into whether the six cases of sickness are linked to the same parasite found in the water treatment works at Mynydd Llandegai.

Test samples have been sent to a reference unit and the NPHSW said it is unlikely that those results will be available until the end of next week.

It is the third time within three years that the water company has issued warnings over the supply in north Wales.

For several days in mid-August, almost 5,000 people in the Tywyn, Aberdyfi, Bryncrug, Rhyd-yr-Onen and Brynglas areas of Gwynedd were also advised to boil their tap water after the quality was affected by heavy rainfall.

In November 2005 a total of 231 people were left ill in Gwynedd and Anglesey after cryptosporidium affected a Snowdonia reservoir.

The water company was fined £50,000 after admitting supplying unfit water in that case.

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