The former director of the UN's oil-for-food programme in Iraq has been accused of taking kickbacks by a panel investigating corruption allegations.
Mr Sevan says the charges against him are false
In its third report, the UN-appointed Volcker panel said Benon Sevan took nearly $150,000 in cash bribes.
UN chief Kofi Annan lifted Mr Sevan's UN immunity at the panel's request.
Meanwhile, a former UN procurement officer has pleaded guilty to accepting hundreds of thousands of dollars in bribes from UN contractors.
The move came hours after investigators said Alexander Yakovlev had receiving almost $1m in bribes outside of the oil-for-food programme.
They also accused him of soliciting a bribe from a contractor who was involved in the oil-for-food programme.
Mr Yakovlev pleaded guilty to conspiracy, wire fraud and money laundering, a federal prosecutor said.
Mr Annan had waived Mr Yakovlev's diplomatic immunity earlier on Monday.
The secretary general said that he was deeply concerned by the conclusions reached by the inquiry.
'Money from aunt' claim
The oil-for-food programme was set up in 1996 to allow the then Iraqi leader Saddam Hussein to sell limited amounts of oil to buy humanitarian goods and ease the effects of sanctions.
But since his overthrow in April 2003, the programme has been hit by allegations that the Iraqi government, politicians and UN officials from several countries illegally profited from the programme.
The investigating panel, headed by former US Federal Reserve chairman Paul Volcker, said that Mr Sevan appeared to have received bribes of $147,184 from December 1998 to January 2002.
In its first report in February, Mr Volcker's three-member panel accused Mr Sevan of "ethically improper" behaviour.
They said he had tried to allocate oil sales from Iraq to one company, and questioned $160,000 (£89,000) in cash that Mr Sevan said he received from his aunt between 1999 and 2003.
Following that report, both Mr Sevan and another official were suspended.
Mr Sevan quit the UN on Sunday, after his lawyer said the charges against him were false.
Despite retiring last year, Mr Sevan had been retained on the UN payroll on a nominal annual salary of $1 for the duration of the investigation.
Mr Yakovlev had resigned from the UN earlier this year.
In his resignation letter on Sunday, the Nicosia-born Cypriot insisted he was innocent of any charges that would be made against him.
"The charges are false and you, who have known me all these years, should know they are false," Mr Sevan, who worked with the UN for four decades, wrote to Mr Annan.
"I fully understand the pressure you are under..., but sacrificing me for political expediency will never appease our critics or help you or the organisation," he wrote.
A separate investigation into the oil-for-food programme is also being carried out by the US Senate's Permanent Subcommittee on Investigations, looking into Iraq's alleged allocations of oil to politicians in the UK, France, Russia and elsewhere.