A similar length of private sewerage pipes connected to individual homes is inspected only when there is a complaint because a problem is apparent, such as a blockage or collapse with raw sewage in the yard or garden.

Rats escape sewers

But the poor quality plastic and pitch fibre often used for private sewers and drains are more susceptible to damage, including rats' gnawing.

This allows raw sewage and rats to escape. Once above ground, the rats quickly flourish.

The report also lists climate change, more fast food outlets and litter as contributory factors.

'Sewers not crumbling'

Stephen Battersby, author of the report and founder of CROSS, said: "We're already seeing the beginnings of the disintegration of the sewerage system, as a direct consequence of the situation being ignored for so long by everyone, from the government down."

A spokesman for the industry body Water UK said "We reject any suggestion that our industry's sewers are crumbling or causing dangers to public health.

"On the contrary, all the water service companies have extensive sewer maintenance and replacement programmes in place.

�1,500m invested in sewerage

"All the evidence shows that the condition of the sewers is improving in the same way as the water mains are.

"Last year alone, over �1,500 million was invested in the sewerage service, of which nearly �600 million was spent on maintaining and improving the network itself in terms of sewers, storm overflows and in-line pumping stations.

"Around �3,000 million has been invested specifically in this area over the last six years.

"Whilst it is accepted that more needs to be done, decades of under-investment and neglect cannot be put right in just a few short years."