Tony Blair will express his "deep sorrow" today for Britain's role in the slave trade but will stop short of demands for a full apology.
The Prime Minister describes the slave trade, which Britain helped to ban, as a "crime against humanity" in an article for the black magazine New Nation.
His decision to say sorry for slavery comes after a lengthy cabinet debate and is intended to coincide with the bicentenary next year of the legislation pioneered by William Wilberforce in 1807 to abolish the slave trade.
He says: "It is hard to believe that what would now be a crime against humanity was legal at the time.
"Personally, 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 it ever happened, that it ever could have happened and to rejoice at the different and better times we live in today."
The Prime Minister's condemnation is intended to clear the air before the Government sets out its plans to commemorate the bicentenary.
John Prescott, the Deputy Prime Minister, has been drawing up ideas for the 25 March anniversary including the possibility of a "statement of regret" over Britain's involvement. The ports of Hull, Mr Prescott's constituency, where Wilberforce was the MP, Liverpool and Bristol have been included in cabinet committee discussions with the BBC about the plans to mark the Bill that ended slavery two centuries ago. Liverpool's ships transported about one million slaves from west Africa to the New World.
Lottery money is being put aside for the events that will include exhibitions at museums and art galleries across the country. Britain is also sponsoring a UN resolution reinforcing opposition to slavery.
"The Prime Minister wanted to make a statement that outlines his views on the slave trade before we get to 2007 for the focus to be on the commemoration," said a senior Whitehall official.
Oona King, the former MP for Bethnal Green and Bow, said: "The Prime Minister is stopping short of a full apology, mainly because it leaves the state open to claims for reparations." She added: "I recognise I am not saying they should be paying people like me, whose ancestors were slaves."
A written ministerial statement to Parliament is due this week, setting out the government plans to commemorate the date. Cabinet minister Baroness Amos let slip the details of Mr Blair's statement by carrying her hand-written notes from a cabinet committee meeting in front of the cameras. Her notes contained the phrases "get it out of the way" and "do it before end of the year".
Another comment picked out from Baroness Amos's notes was "prepared to go further than being asked to" - prompting speculation that Mr Blair might in fact issue a full apology.
There will be cross-party support for Mr Blair's remarks but radio phone-in shows were jammed with callers protesting at Mr Blair's apology, some saying that he should be apologising for the Scottish clearances that forced many Scots to Canada.
In the past, Mr Blair has also apologised for the British failure to do more to alleviate the pain and suffering over the potato famine in Ireland.
* POTATO FAMINE
In 1997 Tony Blair said sorry for Britain not doing more to relieve suffering from Ireland's 19th-century potato famine.
This year some 300 First World War soldiers shot for refusing to fight (many of whom were shell-shocked) were pardoned.
In 1995, the Queen officially apologised to the largest Maori tribe in New Zealand for the devastation wrought on their land in the 1860s.
* AMRITSAR MASSACRE
In 1997 the Queen visited Amritsar in the Punjab, scene of a massacre of up to 1.200 people in 1919. She said it was "distressing", and said: "History cannot be rewritten, however much we might sometimes wish otherwise."