Former human rights lawyer Phil Shiner has pleaded guilty to three counts of fraud, linked to claims made against British Iraq War veterans.
Shiner, 67, appeared at Southwark Crown Court on Monday where he entered the guilty pleas, the National Crime Agency (NCA) said.
The former boss of Public Interest Lawyers was struck off by the Solicitors Regulation Authority in 2017 for pursuing false torture and murder allegations against British troops.
The head of the NCA's International Corruption Unit said the conviction "is a milestone".
The three charges relate to allegations of fraud offences over legal aid claims made in 2007.
Then, Shiner, from Birmingham, made an application to the Legal Services Commission, seeking up to £200,000 of legal aid funding for his firm to represent clients including Khuder Al-Sweady, in an application for judicial review.
It was claimed that his nephew, Hamid Al-Sweady, was unlawfully killed while in the custody of British troops at military base Camp Abu Naji.
A lengthy inquiry into wider allegations of abuse at the hands of British soldiers established "beyond doubt" that all the most serious allegations had been found to be "wholly without foundation and entirely the product of deliberate lies".
The inquiry report said that Hamid Al-Sweady had been killed "outright" during fighting and was not detained alive.
The NCA said that, in total, Shiner received around £3 million in the value of the contract and that the ensuing Al-Sweady inquiry cost the tax payer £24 million.
The agency said Shiner had failed to disclose that an agent acted on his behalf and had been cold calling and making unsolicited approaches to potential clients in Iraq.
The second count of fraud was levelled after he was found to have not disclosed he was paying referral fees - a practice not permitted while obtaining a legal aid contract.
He was also convicted for providing a witness statement to the Commission in support of his application, which was again obtained via an unsolicited approach.
Because this information was not disclosed, Shiner was able to gain a valuable legal aid contract to pursue the judicial review, the agency said.
Andy Kelly, head of the NCA's International Corruption Unit said: "This conviction is a milestone in what has been a thorough and complex domestic and international investigation."
"Shiner's actions resulted in untold pressure and anxiety on members of the British Armed Forces, pursuing legal challenges funded through dishonest actions," he said.
Shiner will be sentenced on 2 December.