A water tank infected with cryptosporidium has been drained and will be cleaned on Saturday, South West Water has said.
About 16,000 households in the Brixham area of Devon have been told to boil their drinking water following 46 confirmed cases of the disease.
Cryptosporidiosis is caused by a tiny parasite and can lead to vomiting, stomach cramps and watery diarrhoea lasting about two weeks.
More cases are expected to be confirmed due to a delay in developing symptoms – and about 100 more people around Brixham were reporting signs of it on Friday.
South West Water believes the parasite probably entered supplies through a damaged pipe in a field containing animal faeces.
A contaminated water tank at Hillhead reservoir, where cryptosporidium was detected, was drained overnight and will be “thoroughly cleaned” on Saturday before refilling starts, South West Water said.
However, it is still unclear when supplies will be safe again as the company stressed it would “only lift the boil water notices when it is absolutely safe to do so”.
The local MP has warned “heads are going to roll” over the incident.
Tory MP Anthony Mangnall, whose constituency includes Brixham, told LBC: “This is such a serious matter that yes, I think heads are going to roll over this.”
He claimed the supplier had been too slow to issue its safety alert.
Mr Mangnall said: “From starting this week with a denial from South West Water that it was anything to do with them, delaying the fact that the boil water notice came in – meaning thousands of people used the water network – to then issuing it on Wednesday, and there are a lot of people who are very ill.”
He called it an “absolutely disastrous week” and said locals were furious.
South West Water has said it’s “deeply sorry” and that it’s been “working tirelessly” to identify the source of the problem and fix it.
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Liberal Democrat leader Sir Ed Davey accused the government of not doing enough to hold water companies to account.
He told Sky News the firms were “putting profit over the environment, over public health” after multiple cases of sewage being released into rivers and seas.
“We [Liberal Democrats] wouldn’t wait for fines for pollution,” Sir Ed said.
“There should be a sewage tax on the profits of these water companies so we can get the money now.
“There needs to be much tougher regulation… and we may need to look at restructuring the whole water industry.”
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It comes as dozens of protests over sewage releases were planned for Saturday.
Surfers Against Sewage is promoting “paddle-out” demonstrations at 30 locations, with Olympian Dame Kelly Holmes among those at the event in Brighton.