(Reuters) -Britain is set to formally classify Russian mercenary force Wagner Group as a terrorist organisation as a way of increasing pressure on Russia, The Times newspaper reported on Tuesday.

Wagner mercenaries have spearheaded Russia's months-long assault on the Ukrainian city of Bakhmut.

The Home Office has been building a case for two months and proscription was due within weeks, the newspaper reported, citing a government source.

The British government declined to comment on its plans.

"The UK sanctioned the Wagner Group in March 2022. While the government keeps the list of proscribed organisations under review, we do not comment on whether a specific organisation is or is not being considered for proscription," a government spokesperson said.

Proscription means it would be a criminal offence in Britain to belong to or promote the group, arrange or address its meetings, and carry its logo in public.

There has been no evidence that Wagner or individuals linked to the group are operating in Britain since the war in Ukraine started, the report said.

However the newspaper said, citing a government source, there had been "suspicions" the group helped move money out of Britain after financial sanctions were imposed on Russian oligarchs and allies of Russian President Vladimir Putin,

Bakhmut has been under Russian attack for more than nine months, with Wagner Group mercenaries leading repeated attempts to advance on what was once a city of 70,000.

The group's leader, Yevgeny Prigozhin, said in a social media message on Monday that his troops were beginning to receive ammunition needed to press their advance.

(Reporting by Jyoti Narayan and Mrinmay Dey in Bengaluru and Sachin Ravikumar in London; Editing by Chris Reese and Sandra Maler)