Mikhail Fridman’s investment firm Letter One last week made a €1.5bn (£1.1bn) bid for the company, which owns 12 active oil and gas fields in the North Sea.
However, the Department of Energy and Climate Change (DECC) is trying to block the sale of the British assets, over concerns surrounding potential sanctions on Russia. Letter One said it would consider legal action if the UK continued to interfere with its bid.
Secretary of state Ed Davey said he would be “willing to consider further representations from Letter One”.
Letter One said: “We have provided DECC with a detailed response and continue to engage with the department. We will leave no stone unturned to find a solution.”