It said the blaze broke out on Wednesday while planned maintenance was taking place at the site near Ashford in Kent.
Prices of natural gas, which have already been at record highs in recent weeks, soared more than 18 per cent at the news.
British electricity prices meanwhile jumped by 19 per cent to £475 per megawatt hour on Wednesday.
The IFA1 interconnector had been used to import electricity, generated largely by nuclear power, from France.
Interconnectors are subsea high-voltage cables, through which Britain imports and exports electricity with countries such as Belgium, France, Denmark and Ireland. They have helped to balance the electricity system in recent years as countries move away from fossil fuels to more weather-dependent sources of energy.
“Our investigation is ongoing,” a spokesperson for the National Grid said. “We will update the market with any changes as necessary.” It is as yet unknown what started the fire.
Kent Fire and Rescue Service said that more than 10 fire engines were called out to tackle the blaze in the early hours of Wednesday morning.
Glenn Rickson, head of European power analysis at S&P Global Platts Analytics, said the fire “couldn’t come at a worse time for the UK”.
Rajiv Gogna, a partner at LCP Energy Analytics, said: “Unfortunately we are going to see more high and volatile prices which is going to feed through to consumers.”
Charities have warned that many households will be forced to ration their heating this winter as multiple factors continue to push up energy prices across Europe.
UK prices have hit new highs in recent months due to a global gas shortage, a surge in demand following coronavirus restrictions, and a slump in wind farm generation in the North Sea.
Gas stocks in Europe have also taken longer to replenish this year after a curb in supplies from Russia.
Gas production has also been 28 per cent lower in the UK so far this year because companies have been carrying out maintenance that was delayed due to the pandemic.
Ruth London, of the campaign group Fuel Poverty Action, told the Financial Times that higher bills for families would be “devastating” especially “when you get people who are already counting pennies and who are already rationing their heating”.
The IFA2 interconnector, a second link between Britain and France, is still operating at full capacity and was not affected by the problem.
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