The announcement came a day before international talks in London aimed at recovering assets which may have been looted under the regime of deposed pro-Moscow president Viktor Yanukovych.
Britain's Serious Fraud Office (SFO) said it had "opened a criminal investigation into possible money laundering arising from suspicions of corruption in Ukraine."
"The SFO has obtained a restraint order freezing approximately USD 23 million of assets in the UK in connection with this case."
It said it could not provide more details for reasons of confidentiality.
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Senior government officials, prosecutors and representatives from financial centres and international organisations will also attend, Britain's Home Office said.
Several countries are helping Ukrainian-led investigations into alleged corruption and money laundering by members of Yanukovych's government.
Swiss authorities have ordered a freeze on the assets of both Yanukovych and his multi-millionaire son Oleksandr, as well as 18 other former ministers and officials.
Yanukovych was ousted in February following a series of massive protests after he decided to scrap an agreement with the European Union in favour of closer ties with Russia.