Prime Minister Tony Blair has said he feels "deep sorrow" for Britain's role in the slave trade.
Slavery had been 'profoundly shameful', Mr Blair said
In an article for the New Nation newspaper, the prime minister said it had been "profoundly shameful".
But Mr Blair stopped short of issuing a full apology, which some commentators have demanded.
The government is reportedly setting out its plans for next year's bicentenary of the abolition of the slave trade.
Esther Stanford, of the Pan African Reparation Coalition, said all countries that had ever been involved in slavery should give a full apology.
"An apology is just the start - words mean nothing," she told BBC News.
"We're talking about an apology of substance which would then be followed by various reparative measures including financial compensation."
She said: "If we do not deal with this now it is tantamount to saying that you can commit crimes against humanity, against African people and get away with it."
'Crime against humanity'
Deputy Prime Minister John Prescott has been drawing up ideas for the 25 March anniversary, including the possibility of a "statement of regret" for Britain's involvement.
He has already ruled out a formal apology.
Mr Blair said: "It is hard to believe what would now be a crime against humanity was legal at the time.
"I believe the bicentenary offers us a chance not just to say how profoundly shameful the slave trade was - how we condemn its existence utterly and praise those who fought for its abolition - but also to express our deep sorrow that it could ever have happened and rejoice at the better times we live in today."
The statement is due to appear in New Nation, a newspaper aimed at the black community, on Monday.
Culture minister David Lammy said that he "did not want to get into a blame fest" but wanted next year's events to celebrate the people who abolished the trade and commemorate those who died.
"In Tony Blair's statement today he recognises that there is a legacy from this period of history in Africa and that there is a legacy in relation to black people living here in Britain."
He stressed that the Labour Party had always focused on equality and would continue to fight against modern slavery.
"Tony Blair has gone further than any other leader of any western democracy," Mr Lammy said.
"He has struck the right balance between providing for the future, commemorating the past and moving forward as a multi-ethnic nation."
Activist Paul Stephenson told the BBC: "The prime minister could have gone further, but nevertheless it is a step in the right direction."
Richard Dowden, director of the British Royal African Society, said the bicentenary would be a chance for Britons to "acknowledge slavery as part of their history".
"This happened at a time when Britain was becoming the Britain that we have today," he said.
"It was the beginning of the industrial revolution, it was when Britain began to rule the world and many of our national heroes were deeply implicated in it.
"Nelson, for example, called the abolitionist movement a damnable doctrine - he fought to protect the slave trade."
A written ministerial statement to Parliament is expected this week, setting out the government's commemoration plans.
In February, the Church of England General Synod voted to apologise to the descendants of victims of the slave trade.