The SEAL found earlier this week 17 miles inland from the coast is proof the River Mersey is clean.

The seal washed up in a farmer’s field after swimming up the river, which used to be one of the most polluted waters in Europe.

United Utilities said today that the animal’s survival is testimony to the millions of pounds spent cleaning up the river over the past 20 years.

The company completed the MEPAS scheme in the 80s, removing millions of gallons of untreated sewerage from entering the river.

The 29km super-tunnel collected 28 separate raw sewerage outfall pipes from Crosby to Garston and transported it to a new Liverpool Treatment works on Sandon Dock.

Since the clean-up, animals have been gradually returning to the Mersey.

Salmon were discovered recently, and four species of owl have also been spotted around the Fiddlers Ferry area.

Recently a new £200 million extension to Liverpool's Wastewater Treatment Works, will keep the Mersey clean for generations to come, and is now just over half-way through its construction.

When complete, the new plant at Wellington Dock will serve around 600,000 Liverpudlians, taking away their sewage and treating it to the highest standards.

A United Utilities spokesman said: “In the past, the seal would have died. The fact it survived is real evidence that the River Mersey is much cleaner.

"This has been a wonderful Christmas present to everyone who has been involved in the scheme.”

The seal was found yesterday on Tuesday in Newton-le-Willows, sparking a major operation by the emergency services.

It is thought he ended up there after trying to catch fish along the River Mersey The rescued animal was herded into a RSPCA trailer with brooms and fences.