United Utilities (UU) said the problem for people living in Lancaster and Morecambe was likely to continue until Thursday.

Householders were alerted to the problem on Tuesday, following the detection of the fault at the treatment works in Scotforth.

It means some of the water in UU's tanks has not been fully re-processed before going back in to the domestic supply, but the company said people should not be alarmed.

'Problems unlikely'

Area manager Stuart Atkinson said the move was a "precautionary measure".

He told BBC Radio Lancashire: "We discussed the matter with the local health authority and we've agreed to ask our customers to boil water for a short time as a prudent step, until we can get our supplies back to normal.

"It's a small amount of water that hasn't been treated to our usual standards and will be sat in our storage tanks at the moment.

"It is unlikely customers will have any problems, but if they do have particular concerns then they should contact their local GP."