The fighters have joined “pro-Russian units” on the battlefield, the Ministry of Defence (MoD) said.
The MoD said: “The exact status of the redeploying personnel is unclear, but it is likely individuals have transferred to parts of the official Russian Ministry of Defence forces and other [private military companies].”
Reports suggest that “a concentration of Wagner veterans” are fighting near the key town of Bakhmut, UK officials said.
“Many will be familiar with current front line and local Ukrainian tactics, having fought over the same terrain last winter,” they said.
Former Wagner boss Yevgeny Prigozhin, who was forced into exile in Belarus following the unsuccessful mutiny, died in a plane crash last month.