A UK aide to one of Vladimir Putin’s “closest” oligarchs has agreed to forfeit more than £750,000 after a two-year investigation into sanctions dodging.

Cash belonging to Petr Aven – a tycoon closely linked to Russia’s president for about three decades – had previously been frozen in 2022, shortly after the businessman was sanctioned by the UK for supporting the Kremlin in the wake of Russia’s invasion of Ukraine.

Attempts to move funds and use them for the benefit of the Russian billionaire was in breach of sanctions law, the National Crime Agency (NCA) alleged. Some of the cash paid the salaries to more than 20 members of Aven’s household staff, while the oligarch is said to have benefited from the proceeds from a sale of a Bentley Bentayga worth £160,000 in late March 2022, Westminster magistrates court heard on Monday.

The court then ratified an agreement between the NCA and Aven’s estate manager, Stephen Gater, for the forfeiture of £783,827.34.

The cash originally arrived in the UK in March 2022 as part of a £3.7m transfer from an Aven trust in Austria, that had taken several days to complete. The transaction had previously emerged as part of Cyprus confidential – an international media investigation into the offshore services industry in Cyprus by a team of reporters, including the Guardian.

The £3.7m transfer was initiated only hours before the public notice that Aven had been placed under EU sanctions on 28 February 2022 – and two weeks before similar restrictions were imposed by the UK government.

When placing the oligarch under sanctions on 28 February 2022, the EU said: “Petr Aven is one of Vladimir Putin’s closest oligarchs … He is one of approximately 50 wealthy Russian businessmen who regularly meet with Vladimir Putin in the Kremlin. He does not operate independently of the president’s demands.”

Aven has called the restrictions “spurious and unfounded”.

The NCA said the forfeiture was the first time it has seized – rather than frozen – sanctioned funds under the Proceeds of Crime Act. The agency had previously demanded that Aven forfeit as much as £1.1m, according to the court order.

The NCA had also accused Aven of being a “pro-Kremlin oligarch” who had acquired his wealth in the last days of the Soviet regime, an allegation denied by his lawyers.

The billionaire is separately seeking to lift the UK sanctions against him, according to reports. Aven’s personal beliefs put him in “a position of opposition to the current regime,” his lawyer told the court.

A spokesperson for the NCA said the forfeiture “demonstrates the NCA’s commitment to enforcing the UK’s sanctions regime, and to recovering money held unlawfully for the benefit of the public purse”.

Lawyers for Aven did not respond to requests for comment.