Russian billionaire Mikhail Fridman must sell his North Sea gas fields within six months, the UK has ordered.
His LetterOne Group acquired the assets as part of a €5.1bn acquisition of the oil and gas arm of Germany's RWE.
UK energy secretary Ed Davey fears production from the gas fields could shut if further sanctions are imposed on Russia over Ukraine.
Mr Fridman has previously threatened legal action to stop an enforced sale.
In a statement on Monday, the UK Department of Energy & Climate Change said it would revoke the North Sea licences unless there was a "change of control" within three months.
However, Mr Davey added that this could be extended to six months.
"This decision was taken after a thorough review of all relevant information as well as obtaining cross-Government views," the statement said.
The gas fields bought by Mr Fridman account for about 3%-5% of UK supplies, sizeable enough for the UK to consider them of strategic importance.
The billionaire, ranked by Forbes as the world's 68th richest person, is one of the energy sector's most influential dealmakers.
His oil and gas interests stretch from Algeria to Poland, Libya to Norway.
But with oil and gas prices depressed, and concern that the North Sea sector needs considerable investment, it is unlikely that there will be a long queue of potential buyers for the assets.
The billionaire had previously considered putting the North Sea fields into a separate legal entity that would shield them from the impact of further sanctions.
A spokesman for Mr Fridman's LetterOne declined to comment.