Desperate to halt a probe into his finances, former Delta State Governor James Ibori tried to bribe anti-corruption boss Nuhu Ribadu in 2007 with $15m in cash in a bag so heavy one man alone could not lift it, Ribadu told a London court on Thursday.

Reuters quoted Ribadu as telling the court that he pretended to take the bribe because he wanted the cash as evidence to use against Ibori in a prosecution, but rather than keep the money for himself he had it taken straight to the Central Bank of Nigeria to be kept safe in a vault.

Ibori was governor of oil-producing Delta State in southern Nigeria from 1999 to 2007. In 2012, he pleaded guilty at London’s Southwark Crown Court to 10 counts of fraud and money-laundering and was jailed for 13 years.

He is the most senior Nigerian politician to be held to account for the corruption that has for decades held back Africa’s most populous nation and top oil producer.

Ribadu told the court that about $1 billion flowed from federal government accounts into Delta State coffers during Ibori’s eight years in power, and he estimated Ibori had stolen or wasted more than half of that amount.

The charges to which Ibori pleaded guilty amount to the theft of about $80m, but British prosecutors say that was only part of his total booty, which was kept hidden via a complex web of shell companies, offshore accounts and front men.

Ribadu, who was chairman of Nigeria’s Economic and Financial Crimes Commission (EFCC) from April 2003 to December 2007, was giving evidence at a confiscation hearing in which prosecutors are seeking court orders to have Ibori’s assets seized.

Under Nigeria’s constitution, state governors enjoy immunity from prosecution but are limited to two terms in office. With the end of his second term looming in April 2007, Ibori was worried the EFCC were planning to prosecute him, Ribadu said.

“He was very desperate to terminate the investigation,” he told the court.

In late April 2007, a meeting was arranged between the two men at a “neutral place”, the house of Andy Uba, a close associate of outgoing President Olusegun Obasanjo.

Ibori arrived at the house with several members of his staff and a very large black sack containing $15m in cash. Ribadu said he watched as two of Ibori’s men lifted the heavy sack and handed it over to his own EFCC staff. “It was a bag that an individual could not carry alone,” he said.

The EFCC men drove the bag to the central bank where the money was counted and boxed into smaller containers. The court was shown photographs of the boxes of cash.