Tymoshenko is serving a seven-year prison sentence following a conviction on charges she abused her authority when she helped broker a natural gas deal with Russian energy company Gazprom while serving as prime minister in 2009.
She announced from prison last week she was waging a hunger strike to protest against her treatment. She claims she was roughed up by prison officials, though authorities at the penal colony housing the former prime minister denied the allegations.
British Minister for Europe David Lidington, in a statement, said he was appalled by the reports.
"I was dismayed to see reports that former Ukrainian Prime Minister, Yulia Tymoshenko, had been physically abused by prison staff," he said in a statement.
Kiev said it was investigating the claims.
Meanwhile, at least two dozen people were injured in a series of bomb explosions in the Ukrainian city of Dnepropetrovsk, Tymoshenko's home town. Four blasts were reported in the city around noon Friday.
The British Foreign and Commonwealth Office issued an advisory shortly after the attacks. The threat of terrorism, the warning read, was low though British nationals in Ukraine were urged to be on the alert.
No travel restrictions were associated with the alert. Ukraine doesn't have a history of terrorism and no group claimed responsibility for the attack.