James Ibori, former governor of Delta State, Nigeria speaks with a reporter,16 July 2003 James Ibori is accused of using stolen money to fund his lavish lifestyle
James Ibori, a former governor of one of Nigeria's oil-producing states, has pleaded guilty in a UK court to 10 counts of money-laundering and conspiracy to defraud.
British police accuse him of stealing $250m (£160m) over eight years.
The prosecutor called him a "thief in government house".
Mr Ibori, once seen as one of Nigeria's wealthiest and most influential politicians, was arrested in 2010 in Dubai and then extradited to London.
As his trial at London's Southwark Crown Court was about to begin, Mr Ibori changed his plea to guilty and admitted stealing money from Delta state and laundering it in London through a number of offshore companies.
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“ We will now be actively seeking the confiscation of all of his stolen assets so they can be repatriated for the benefit of the people of Delta state”
Paul Whatmore Metropolitan Police
Prosecuting QC Sasha Wass said Mr Ibori, 53, had "tricked" his way into becoming Delta state governor, by giving a false date of birth and claiming he had no criminal record.
"He was never the legitimate governor and there was effectively a thief in government house. As the pretender of that public office, he was able to plunder Delta state's wealth and hand out patronage."
He is due to be sentenced on 16 April.
Mr Ibori's wife, Theresa, his sister, Christine, his mistress, Udoamaaka Okoronkwo, and his London solicitor, Bhadresh Gohil, have all been convicted of money-laundering.
The BBC's Chris Summers, who was in court, says their convictions could only be reported on Monday after reporting restrictions were lifted.
Nigeria's Economic and Financial Crimes Commission (EFCC) had asked the UK's Metropolitan police to look into the ex-governor's financial affairs.
"The vast sums of money involved were used to fund Mr Ibori's lavish lifestyle," Detective Inspector Paul Whatmore, the officer in charge of the investigation, said.
"We will now be actively seeking the confiscation of all of his stolen assets so they can be repatriated for the benefit of the people of Delta State."
He said that the money Mr Ibori stole should have been used to pay for sanitation, power supplies and healthcare for some of the poorest people in the world.
In 2007, a UK court froze assets allegedly belonging to him worth $35m.
His annual salary as Delta state governor was less than $25,000.
He had already left the UK when his assets were seized but he was arrested in Nigeria later that year.
The charges were later dropped but he was then re-arrested in Dubai on a British warrant.
Under Nigeria's federal system, state governors enjoy huge powers and control budgets larger than those of many West African countries.