“The tragedy is, it still exists in the world today.”

Newton’s conversion began in May, 1748, after he and his crew were returning to England, via the Atlantic triangle trade route, and encountered a severe storm.

He awoke in the middle of the night, fell to his knees and prayed. As he said the Lord’s Prayer, the water began to leave the ship and the storm calmed.

But the hymn’s famous melody was only joined to Newton’s lyric in 1835, after it was taken up by the descendants of slaves who Newton transported from Africa to America in his early life.

The artwork was created by artist Stephen Broadbent, who was responsible for a wide variety of other public artworks around the city of Liverpool, and was unveiled on his 48th birthday.

His installation, created from pierced steel plates, represents the three continents of the triangular slave trade – Africa, Europe and North America.

Mr Broadbent said: “This story of how lyric and melody came together was a wonderful story to try and tell.”

The hymn’s popularity grew after it was taken up by both sides in the American Civil War.

It is also considered a Cherokee national anthem, and many contemporary Native American musicians have recorded it.

In recent years, the song has become popular in the USA with drug and alcohol recovery groups, particularly Christian ones, at celebrations of how they “once were lost, but now are found”.

It has also become known as a favourite with supporters of freedom and human rights, because many assume it to be Newton’s testimony about his slave trading past.

The unveiling of the installation yesterday also marked the completion of work on the new Pier Head ferry terminal building.

Merseytravel chief executive Neil Scales said: “We are very proud of what we have achieved with this building and our role in stimulating the environment with public artwork.”