The Wagner Mercenary Group has likely lost access to Russia's penal system following a public feud with the Kremlin, the Ministry of Defence (MOD) has said.
In the latest defence update, the MOD said Russia's MOD has "ramped up a scheme to recruit Russian prisoners to fight in Ukraine" this year.
"It is likely that up to 10,000 convicts signed up in April 2023 alone," the MOD said.
"From summer 2022, prisoners were the key pool of recruits for the Wagner Group private military company's operations in Ukraine."
The Wagner Group is a mercenary group that has been fighting alongside Russian forces since Moscow invaded Ukraine in 2022.
The MOD added the Wagner Group has "likely lost access to the Russian penal system in February 2023 when its public feud with the [Russian] MoD was escalating".
"The [Russian] MoD's prisoner recruitment campaign is part of a broader, intense effort by the Russian military to bolster its numbers, while attempting to avoid implementing new mandatory mobilisation, which would be very unpopular with the Russian public," the MOD said.
It comes as the UK Government are reportedly set to make the Wagner Group a proscribed terrorist organisation.
Proscription would make it a criminal offence to join Wagner, encourage support for it, display its logo in public or attend its meetings.
The Wagner Group have been fighting alongside Ukrainian forces in Eastern Ukraine, with a particular focus on Bakhmut.
The Wagner Group is controlled by founder Yevgeny Prigozhin, an oligarch with reportedly close links to President Putin, and has been used by Russia throughout their invasion of Ukraine.
Earlier this month, Mr Prigozhin branded Russian commanders cowards and accused them of starving his troops of supplies as they were thrown into what he called the meatgrinder of Bakhmut.
He went on to claim the Russian military was jealous of his men's success and wanted victory for themselves, saying they'd cut supplies to the Wagner Group in order to make sure they failed.