British Prime Minister Boris Johnson on Friday backed England rugby fans singing "Swing Low, Sweet Chariot" at matches, despite suggestions it could be banned because of its links to slavery.

The song, reportedly written by an American slave in the 19th century, is thought to have been first sung in an English rugby context by supporters of junior clubs in the 1970s.

It was then heard at Twickenham, English rugby union's London headquarters, in 1987 when aimed at Martin Offiah, nicknamed "Chariots" after the Oscar-winning film, "Chariots of Fire".

But it really found favour with England fans at Twickenham the following year when another black player, Chris Oti, scored a hat-trick against Ireland.

The Rugby Football Union (RFU), however, said on Thursday it was looking into whether the song should still be sung, because "many fans have no awareness of its origins or sensitivities".

The statement came in the wake of widespread anti-racism protests which have focused renewed attention on Britain's colonial past, in particular those who profited from slavery.

Johnson, a rugby fan, was asked about the RFU assessment of the terrace chant, and said he was more interested in why more fans did not know all of its words.

"You go, 'Swing Low, Sweet Chariot, coming for to carry me home', and then it all dies out. How does it go on? That's my question," he told reporters.

"I certainly don't think there should be any sort of prohibition on singing that song. My curiosity is why don't people seem to know the rest of it."

Johnson has previously made a strong defence of his hero Winston Churchill, after a statue of the wartime leader was defaced with the word "racist" during one recent protest.

There have been calls across Britain for statues of colonial figures to be removed, after protesters toppled a monument to a 17th century merchant and slave trader in Bristol.

The debate within rugby comes at a time of questioning about diversity within the sport.

Forward Maro Itoje, one of several black and mixed race players in the England squad, said fans were not singing with malicious intent, even if the song's history was complicated.

Offiah said he was aware of the connotations but did not think it was racist. Instead, he said there should be more education about the song's origins and history.

"We can use it as a coming together and... as a vehicle to learn more about the black players who have represented England," he told Sky News television.

"So, I think you can put a positive spin on this. I'm not one of those people who thinks history should be a sponge, you know, wiped away.

"History should be seen in its full line, the good and the bad. We learn about it and we move on, and we can still sing the song."

Any RFU ban would be divisive, he said, adding: "I'm all for the free will of the people."afp