Mayor Ken Livingstone has formally apologised for London's role in the slave trade.
Mr Livingstone said slavery was "one of the greatest crimes"
He called on Prime Minister Tony Blair to follow suit by issuing an official apology on behalf of the UK.
"The government's refusal of such an apology is squalid," he said, on the eve of the bicentenary of legislation to abolish the slave trade.
Mr Livingstone urged fellow Londoners to join him in apologising for this "monstrous crime".
He said France, the US state of Virginia, the city of Liverpool, and the Church of England had formally apologised over slavery.
"It will be infinitely better for our country's reputation if that apology is made now justly, frankly and openly," he said, and added: "Delay demeans our country."
Mr Blair has previously expressed "deep sorrow" for Britain's role in the slave trade, a Downing Street spokesman said.
In an article for the New Nation newspaper in November 2006, he said it had been "profoundly shameful".
But Mr Blair stopped short of issuing a full apology, which some commentators have demanded.
Simon Woolley, the director of Operation Black Vote, said the mayor had made a "bold but undeniable statement".
"By apologising, we begin the process of reconciliation and addressing the legacy of this gross act of inhumanity," he said.
"It is important because the legacy of slavery remains with many black people on a daily basis."
The Bicentenary of the Abolition of Slavery Act, which banned the British trans-Atlantic slave trade, is on 25 March.