Trish Tweddle of Anglian Water this morning said the cards had been sent to Kettering by mistake.

Once the all-clear is given, residents receive a green card advising them the situation is normal.

She said the error was made because the company based deliveries on postcodes.

She said: "I can categorically say that Kettering is not affected."

However, anyone who remains concerned can boil their drinking water.

Anglian Water experts are today continuing its work to identify the source of the contamination detected at Pitsford Reservoir.

Company spokeswoman Collette Nicholls said: "We don't know how it got into the water system. It was detected at Pitsford Reservoir and it is crucial we find the source of the infection as soon as possible.

"We discovered it on Tuesday. We carry out more than 1,000 random checks every day in our patch and it was discovered and verified at about 1.30am. Once it was verified it was all systems go."

Ms Nicholls said: "It is imperative that we get the message out as quickly as possible regarding notices to boil water.

The water company is issuing bottled water to people in poor health, the disabled and the vulnerable.

She said it was too soon to give any indication on compensation.

Have you been hit by the tap water crisis? You can post your comments on this page by clicking the blue Register button and following the instructions.

Or you can click here to send us an e-mail.

Alternatively, why not go to our Comment Room to debate the issue.

Buy today's Evening Telegraph for more on the tap water crisis or click here to see how the drama unfolded.