The teaching of Britain's involvement in the slave trade is "patchy" and not enough time is devoted to it despite the subject being on the national curriculum, it has been claimed.
The Department for Education has admitted to Sky News that it has no idea how many schools are actually teaching the subject because it is not compulsory.
Britain played a central role in the trade of slaves until its abolition more than 180 years ago, with cities like Liverpool, Bristol and London key trading ports.
Under the last Labour government the then prime minister Tony Blair controversially expressed "sorrow" for the country's involvement in the slave trade but stopped short of apologising for it.
The release of several high-profile films has brought the issue to the forefront once again.
The British director of Bafta-nominated 12 Years A Slave, Steve McQueen, believes the subject should be compulsory in schools.
The film tells the story of Solomon Northup, a New Yorker kidnapped and forced into slavery.
"The movie is really a way into this subject. It gets people talking about it and sparks debate. The Northup memoirs, upon which this film is based, should be essential reading for all schoolchildren, in my view," he told Sky News.
For well over 300 years, Britain played a leading role in forcing Africans onto slave ships for transportation across the Atlantic Ocean.
It is thought that British ships may have carried as many as 3.5 million Africans to the Americas.
But it is unclear now many children learn about the slave trade and how influential Britain was in it.
Dr Richard Benjamin, from the International Slavery Museum in Liverpool, says teachers can use their resources to teach the subject.
"We work with teachers to help teach this part of the curriculum. But the teaching of the subject is a bit patchy and I don't think that three hours at the museum is enough.
"Some children have no idea that Britain was such a big player in the slave trade until they're in their mid-20s. We should not be getting to our mid-20s to find that out."
A Department for Education spokesperson said: "Unfortunately we do not hold information about how many schools are actively teaching the slave trade.
"The slave trade is covered in the new national curriculum, which will be taught in schools from this September.
"Pupils will learn about the British Empire from 1745-1901 in much greater depth as part of the new more rigorous history curriculum.
"Schools are also free to use other opportunities within the curriculum to teach about the slave trade and as part of Black History Month."
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