The UK is set to designate Russia's mercenary Wagner Group as a proscribed terrorist organisation, it is understood.
The Times reported a Government source had said the move was "imminent" and likely to be enacted within weeks after two months spent building a legal case.
Proscription would make it a criminal offence to join Wagner, encourage support for it, display its logo in public or attend its meetings.
Among other organisations on the Government's proscribed list are the Islamic State, al Qaida and neo-Nazi group National Action.
It comes as Labour demanded ministers formally label the Wagner Group a terror outfit after accusing it of committing "appalling atrocities".
The Wagner Group, led by warlord Yevgeny Prigozhin and made up of contractors and recruited convicts, has been fighting alongside Russian armed forces in areas of eastern Ukraine.
It has been heavily involved in the city of Bakhmut, where the longest – and likely bloodiest – battle of the war has taken place.
Labour said in February it wanted ministers to follow the US’s lead after Washington designated Wagner a "significant transnational criminal organisation".
In a joint statement, shadow foreign secretary David Lammy and shadow home secretary Yvette Cooper said: "The Wagner Group is responsible for the appalling atrocities in Ukraine and across the world.
"No-one in the UK should be allowed to belong to the Wagner Group, support it or promote it."
The push for Wagner to be proscribed came after a Government department reportedly helped its millionaire owner Mr Prigozhin to circumvent UK sanctions to take a British journalist to court in 2021.
The Treasury commissioned an internal review of its processes after it was reported that licences had been issued to allow lawyers to help Mr Prigozhin launch legal action against a Bellingcat reporter in the UK while the Russian oligarch was subject to sanctions.
As a result of the review, the department said the Government was committed to "further targeted changes to the process for issuing legal fees licences that safeguard the sanctions regime against the risk of manipulation and ensure ministers are accountable for OFSI Office of Financial Sanctions Implementation) decision-making".