German utility RWE (>> RWE AG) on Monday finalised the sale of its oil and gas production business DEA UK to Fridman's investment vehicle LetterOne, ending months of uncertainty over whether the 5.1 billion euro (3.7 billion pounds) deal would go ahead.

But the British government has said that it is concerned that production at the fields could be halted if the West imposes more sanctions on Russia over Ukraine, and it therefore wants them sold to a third party.

Ed Davey, Britain's energy minister, wrote to Fridman's firm on Wednesday.

"Ed Davey has given them seven days to explain why the Secretary of State should not now proceed to issue Notices under the Licences to require further changes of control," the department of energy said in a statement.

"Protecting these assets is the Secretary of State’s priority."

(Reporting by Andrew Osborn; Editing by Jane Merriman)