Utility firm Severn Trent plc has suspended two members of staff at its south Wales water testing plant while an investigation takes place.
The decision was taken after a recent audit of the Bridgend laboratory by the Drinking Water Inspectorate (DWI) which identified "deficiencies" in the system.
Testing of drinking water samples for some chemicals has also been suspended.
Severn Trent said its other water testing facilities are not affected.
Rachel Henton, managing director of Severn Trent's analytical services division and the Bridgend laboratory's operations manager Tim Down have been suspended on full pay to ensure no possibility of conflict of interest during the investigation by Eversheds and Ernst & Young, said the water company.
The water testing laboratories at Severn Trent are run separately to Severn Trent Water, which is based in Coventry.
A statement from Severn Trent Plc said: "The DWI has brought to our attention deficiencies in the systems of work and associated analytical quality control within the Bridgend laboratory.
"We have therefore decided to suspend testing for inorganic substances at Bridgend pending completion of an investigation which we have commissioned from Eversheds and Ernst & Young.
"Other testing processes carried out at Bridgend continue to operate as normal.
"The suspension of activities applies only to inorganic testing services provided by the STS AS Bridgend laboratory.
"Our other facilities based in Coventry, Runcorn and Wakefield will continue to operate as normal."
While the investigation is carried out testing that would have been provided normally by Bridgend is being carried out at the analytical services facility at Wakefield in Yorkshire.
No other operations within Severn Trent Services or Severn Trent Water are expected to be affected.
Tony Ballance, director of strategy and regulation at Severn Trent Plc, added: "We have acted decisively to address the apparent deficiencies the regulator has brought to our attention in relation to the testing service we provide to our customers.
"We are committed to ensuring that the highest standards are maintained across our businesses at all times. The arrangements we have put in place will facilitate the ongoing independent audit by the regulator (DWI)".
STS AS is one of the three business groups that comprise Severn Trent Services, the other two being Operating Services and Water Purification.
Severn Trent Water supplies water to reservoirs at Lake Vyrnwy and Clywedog in Powys.