The UK's role in the slave trade is a matter of "deep sorrow and regret", Prime Minister Tony Blair has said.
Mr Blair marked the abolition act at a Downing Street reception
In a statement marking the anniversary of the British act abolishing the slave trade, the PM said it was among history's most "shameful enterprises".
He added the same dedication that led to abolition was needed to tackle the "many forms" of modern day slavery.
It comes as the Archbishop of York has called on the UK to formally apologise for its role in the slave trade.
Mr Blair's comments were heard in a video message at a commemorative ceremony that took place at Elmina Castle in Ghana, built by the Portuguese in 1492, which later became sub-Saharan Africa's first permanent transatlantic slave trading post.
It was also relayed to giant screens at events taking place in the UK and around the former British Empire.
The statement appeared to fall short of demands from campaigners who say he has not gone far enough.
But a Downing Street spokesperson said: "The Prime Minister has made a very strong statement emphasising the inhumanity of the slave trade....
"We recognise that there are some who want a further statement but we believe that we must now look to the future.
"To the legacy for Africa and the fight against poverty and disease, to addressing inequalities in Britain today, and to combating the evil of contemporary slavery like human trafficking."
In the statement, Mr Blair said: "It is right that this anniversary is being marked today here in Ghana's Elmina Castle, the scene of such inhuman abuse, and in cities across the UK - in Liverpool, Hull, Bristol and London which played their part in this deplorable trade.
Baroness Amos is to address the event at the former slave fort
"It is an opportunity for the United Kingdom to express our deep sorrow and regret for our nation's role in the slave trade and for the unbearable suffering, individually and collectively, it caused."
He also paid tribute to the "courage and conviction" of those who campaigned to end the "vile trade" including former slave Olaudah Equiano, church leader Thomas Clarkson and MP William Wilberforce.
Mr Blair said there were also countless men and women, "now forgotten by history, black and white, from across Africa, including Ghana, from Britain and many other countries" whose efforts should be recognised.
He went on to describe forced recruitment of child soldiers, human trafficking and bonded labour as modern forms of slavery.
"We must remember as well that poverty, social exclusion and conflict is at the root of this cruelty," he added.
"There is a great deal more to do. But in recent years, we have also seen great progress."
In January, Mr Blair held a reception at Downing Street to mark the anniversary of the abolition act.
In November, he expressed "deep sorrow" for the slave trade in an 850 word article in the New Nation newspaper and earlier this month, after a meeting with Ghana's president, he said: "We are sorry".
The Archbishop of York, Dr John Sentamu, told the BBC he should go further.
"A nation of this quality should have the sense of saying we are very sorry and we have to put the record straight," he said.
House of Lords leader Baroness Amos also addressed the Elmina Castle event, organised by the British Council and the government of Ghana.
The peer, who is descended from slaves herself, said the trade was one of the UK's most "shameful and uncomfortable chapters".
In the UK, an exhibition entitled Resistance and Remembrance Day - which includes a recorded video message from Nelson Mandela - is being held at London's British Museum.
Campaigners want Mr Blair to make a formal apology
And on Sunday morning, Deputy Prime Minister John Prescott and the Prime Minister of Barbados, Owen Arthur, reopened the Wilberforce House museum in Hull.
William Wilberforce was the parliamentary spokesman for the abolition movement.
In Bristol, which had been one of Britain's busiest slave ports, protesters interrupted a ceremony attended by the Duchess of Gloucester at the city's cathedral.
Church and community leaders were leading sermons when members of a black African campaign group Operation Truth 2007 gathered outside, carrying placards and chanting "not in our name".
"We feel this ceremony is totally inappropriate - 1807 did not bring anything for enslaved Africans," said Jendayi Serwah, 41, from Bristol.
"We are here to offer a different perspective."