"This is one of the most important works by Lowry ever to come to the market and it's one of only two known pieces of work by Lowry to depict Liverpool. The other is a dock scene.
"One of our specialists said, it's been unseen for more than 30 years and it's already iconic."
Liver Building, Liverpool, which was bought at the Lefevre gallery in London in 1963, also shows working ships on the Mersey including tug boats, while people enjoy a stroll on the Wirral riverbank.
The Cunard and Port Authority buildings are also clearly visible.
The record for a Lowry painting is £1.9m paid by the Professional Footballers Association in 1999 for Going To The Match, which showed fans on their way to the former Bolton Wanderers' ground Burnden Park.
The second highest price paid for a Lowry is around £600,000. The Walker Art Gallery has one Lowry painting, The Fever Van, which was one of his many views of Salford.
LAURENCE Stephen Lowry was born in Rushholme, Manchester, in 1887, but lived much of his life in Salford. He died in 1976.
He worked a 9-to-5 job with Manchester's Pall Mall Property Company until retiring in 1952, and painted only in his spare time. He produced more than 10,000 works, ranging from finished oil paintings to hastily drawn sketches.
In 1967 he turned down a knighthood, seeing little point in receiving it after the death of his mother.