Engineers will inspect pipes which take water from the Lake Distrrict

The United Utilities scheme, which the firm says is the biggest of its kind, will see improvements made to about 500 miles (805km) of water mains.

The scheme starts on Thursday and is due to take eight years in total.

Two main aqueducts, taking water from Cumbria to Manchester, will be closed off and emptied to allow engineers to walk through for a close-up inspection.

Thirlmere reservoir, Cumbria, which supplies a large amount of Lancashire's water, will be closed for four weeks in October for some of the work.

Victorian engineers

United Utilities programme manager Alan Duncan said: "The scale of this job is enormous, our customers have never seen anything like this before and it's a first for the water industry in this country."

Six major mains will be cleaned from end to end for the first time.

Mr Duncan added: "This will be the first time we've surveyed these aqueducts from end to end inside since they were brought into service, so in some cases the last people to have stood inside it would have been the Victorian engineers who built it."