Dylan Healy, 22, from Cambridgeshire, eastern England, and Andrew Hill, 35, from Plymouth, south-west England, were captured in southern Ukraine and are being held by Russian-backed separatists, reported pro-Kremlin news agency Tass.
It said the self-declared Donetsk People’s Republic had begun criminal proceedings against the two men on charges of being mercenaries.
This comes four weeks after two other British men, Shaun Pinner, 48, and Aiden Aslin, 28, were sentenced to death by the separatists on similar charges.
The families of Mr Pinner and Mr Aslin said they were not mercenaries but members of Ukraine’s armed forces who had settled in the country. The British government said they were prisoners of war and that the separatist court had no legitimacy.
Mr Healy, one of the men apparently facing new charges, was reported missing in April, when a humanitarian group said it had lost contact with him.
The Presidium Network, a UK non-profit organisation that provides support to communities in crisis, said Mr Healy was one of two aid workers who had been trying to help a family escape from Zaporizhzhia in southern Ukraine.
It said the family was subsequently interrogated by Russian forces, who alleged Mr Healy and his colleague were spies.
The other British national detained with Mr Healy, Paul Urey, 45, was not mentioned in the announcement about the charges against Mr Healy and Mr Hill.
A man identifying himself as Andrew Hill was paraded on Russian television in April, after apparently being captured and accused of being a mercenary.
Tass reported that this was the same man now facing charges and said he had been captured in the southern Mykolaiv region of Ukraine.
Citing a Donetsk source, Tass said Mr Healy and Mr Hill “do not want to testify and refuse to co-operate on their criminal case”.
A statement from Britain's Foreign Office said: "We condemn the exploitation of prisoners of war and civilians for political purposes and have raised this with Russia.
"We are in constant contact with the government of Ukraine on their cases and are fully supportive of Ukraine in its efforts to get them released."
Russia recognised the breakaway region and its neighbouring territory Luhansk as independent shortly before invading Ukraine in February.
Britain has said the separatist courts have no legitimacy and that prisoners are being denied their rights under the Geneva Conventions.
Mr Aslin and Mr Pinner, plus Moroccan prisoner Brahim Saadoun, were sentenced to death last month after being found guilty of mercenary activities and plotting to overthrow the DPR.
British Foreign Secretary Liz Truss called it a “sham judgment”, while Robert Jenrick, a former minister whose constituents include Mr Aslin’s family, said it was akin to a “Soviet-era show trial”.
UK ministers have said they are working with the Ukrainian government to try to get them released. The Kremlin said Britain should deal directly with DPR authorities.
Russia and Ukraine have accused each other of fighting with mercenaries during the four-month conflict, with western intelligence reporting the involvement of the Kremlin-linked Wagner Group.