The Crown actor James Murray has accused a water company of “lying through its teeth” for blaming heavy rain for sewage pollution in a heavily protected river.

The environmental campaigner and fly-fishing enthusiast, who portrayed Prince Andrew in the hit drama, said Southern Water should have invested in its infrastructure long ago to prevent its sewers being overwhelmed by extreme weather.

Instead, sewage has been allowed to pour into the River Test, a chalk stream that is one of the rarest habitats on Earth.

He told Sky News: “Every year they say it is a special year and it won’t happen again. It’s nonsense. They lie through their teeth.

“Yes, it might have been a very damp February, but that doesn’t mean they should be putting raw sewage directly into the river. It is unforgivable.”

Southern Water says its Fullerton wastewater works near Andover has been overwhelmed and to prevent sewage backing up into people’s homes it has had to allow it to flow into the river untreated.

The current regulations allow water companies to discharge raw sewage during extreme rainfall.

But river conservationists say the persistent flow of human waste this winter has resulted in ecological damage.

Sky News saw grey tufts of “sewage fungus” wafting in the river current.

It’s a gelatinous slime of viruses, bacteria and other organisms that grows quickly in water that’s heavily polluted with nutrients. It sucks up oxygen, suffocating life that should be in the river.

Phil Parker, a river keeper who maintains a stretch of the Test, demonstrated the impact by doing a “kick sample”.

He stirred up the riverbed with his feet to collect invertebrates living in the silt.

In a sample from a clean section of the river, there was a huge abundance and variety of life.

But in the area affected by sewage, there was comparatively little.

He said: “The samples are a clear indicator that the sewage is killing our invertebrates.

“It’s killing an ecosystem that’s already fragile anyway with the changing climate and the extreme weather patterns we have coming through.”

The south of the UK had twice the normal rainfall in February, capping a wet winter and raising the water table underground.

Southern Water showed Sky News video of water being forced at high pressure into sewage pipes through cracks and joints.

It says it has lined the public network with a special sheath, that it likens to a sausage skin, to keep water out in future.

But it has only recently begun work on pipes on customers’ property. It could be 2030 before all the pipes are fixed.

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Nick Mills, Director of Environment for the company said: “I understand the frustration, and we share many of their outcomes in what they want to see happen.

“But it is a large, complex issue, and we are in some extreme conditions right now.”

Rivers up and down the country are affected by sewage pollution.

Campaigners are hoping to seize on public anger to turn it into an election issue, and force water companies to clean up their act.