Britain's government on Monday handed Russian-controlled investment fund LetterOne a deadline of six months to sell North Sea gas fields acquired from German power giant RWE.

It comes as LetterOne faces possible sanctions owing to Russia's role in the Ukraine crisis according to London.

RWE last month wrapped up the sale of its oil and gas unit Dea to LetterOne in a deal valued at about 5.1 billion euros ($5.5 billion).

British energy minister, Secretary of State Ed Davey, "proposes to revoke Dea UK's North Sea petroleum licences unless LetterOne arranges for a further change of control of the Dea UK gas fields in the North Sea", a statement by the Department of Energy & Climate Change said Monday.

"This decision was taken after a thorough review of all relevant information as well as obtaining cross-government views.

"Under the terms of the licenses held by Dea UK, Dea UK and LetterOne now have three months to effect a further change of control. However, the Secretary of State has offered to extend this period to six months and he awaits LetterOne's response to this offer," the statement added.

In a release issued in late February, Davey "raised concerns... about the effect that possible future sanctions imposed on LetterOne may have on the continued operation" of fields in the North Sea "and the serious health and safety and environmental risks that may result".