An E.Coli alert at a school means staff and students have had to use bottled or boiled water since September.
Water samples taken at the end of August from Brockington College, in Enderby, revealed traces of potentially lethal E.Coli bacteria.
The source of the problem is thought to be the college's own internal water supply.
However, environmental health officers say there is no danger to staff and students at the site and the use of bottled or boiled water is a "precautionary measure".
E.Coli bacteria can cause diarrhoea, but occasionally serious kidney and blood complications can occur.
A spokesman for Blaby District Council, whose environmental health officers took the water samples, said the college had contacted the authority while reviewing its risk assessments, to "clarify if they were doing everything required in relation to providing safe water".
"As a result, the council offered to undertake some sampling as part of their review," said the spokeswoman.
"Samples of water were taken on August 30, and traces of E.Coli were found."
She said the council issued a notice to the school that it should boil water before using it, to rid it of contaminants.
The matter was then taken over by water company Severn Trent, which is now monitoring the situation.
"There is no danger to students or those who work at the college," said the spokesman.
"Fountains have been turned off, boiled water is being used in the kitchens and bottled water is being provided by the college in the form of water coolers throughout the building.
"Parents were informed there were issues in the early stages and were asked to send children in with bottled water, although it was also provided by Severn Trent on site initially.
"The college has responded positively and done everything asked of it. We are working with Severn Trent to achieve the improvements needed to lift the boil water notice."
A Severn Trent spokesman said: "We discussed the result with the local authority and the health protection unit and a decision was collectively made by both organisations to issue a precautionary boil water notice to the college while an investigation was carried out into the quality of water within the school."
She added: "As part of our investigation, the water supplied to the college by Severn Trent was thoroughly sampled and the results were found to be satisfactory.
"However, one of the samples collected from the college's internal water supply didn't meet the water quality regulations standards."
She said Severn Trent carried out "multiple inspections of the plumbing", including the set-up of the tank system supplying the college with water, and this "identified improvements that could be made to the plumbing system within the school".
Corrective work was carried out by the college to try to resolve the matter, but further sampling confirmed E.Coli was still present, she said.
Therefore, when students return to the college following the half-term holidays, water restrictions will still be in place "until the issue has been resolved".
Head teacher Chris Southall was unavailable for comment, but in a newsletter to parents last month he said he was "frustrated" about the issue.
"We have, unfortunately, unearthed a series of issues concerning the initial set-up of the system, which can only be corrected at weekends to ensure that the college remains open during the week," he said.
E.Coli bacteria is found in the gut of humans and animals. It can be dealt with by chlorine and other disinfectants.
