Porpoise visits the ship canal.
The 5ft long harbour porpoise was first spotted yesterday morning and is now being monitored by marine experts.
A spokesman for the British Divers Marine Life Rescue team said it was quite unusual for a porpoise to travel so far upstream, but it appeared to be in good health.
He said: "When last sighted, it was around 21 miles inland, close to Latchwood Lock in Warrington.
"We are keeping an eye on the situation because there is always a danger it could get trapped in the lock gates or stranded, but so far it seems to be doing fine."
Porpoises normally live in seawater but can survive in freshwater for several weeks without suffering any ill-effects. The animals are now regular visitors to the River Mersey.
VISITOR: This porpoise was seen near Princes Dock