France provided around 5 percent of Britain's electricity last year but flows have also been going in the other direction this winter, after safety checks on several of France's nuclear reactors curbed its power output.

Britain's National Grid, which operates the 2,000 megawatt (MW) Interconnexion France-Angleterre (IFA) with French grid operator RTE, said four of its eight cables had been severed.

"This will result in a reduction of IFA's maximum capacity to 1,000 MW until the end of February 2017," a spokesman for National Grid said in an emailed statement.

He said the company was carrying out an investigation to determine what had caused the damage.

France, which is typically an exporter of electricity, has become more reliant on imports from other countries due to the issues with some of its nuclear plants, leading to volatile prices in Britain and continental Europe.

"The reduced capacity ... is likely to lead to increased volatility and higher UK power prices over January and February 2017 – especially during peak demand periods," analysts at Barclay's Capital said.

(Reporting By Susanna Twidale, editing by David Evans)