The Rough site, a depleted gas field off England's east coast, stopped storing gas in 2017 but was reopened by Centrica in October last year and now provides half of the UK's total gas storage capacity.

Centrica, which has filled Rough with the equivalent of 18 LNG tankers this year, said the site provides enough energy to heat more than 3 million homes every day during winter.

"Customers are struggling with high energy bills, which are driven by international energy prices. Gas storage is vital to ensure the UK can manage demand effectively, keeping prices down," said Centrica CEO Chris O'Shea.

Countries across Europe have been building gas stocks ahead of winter because Russian gas pipeline supply has dwindled since the start of the war in Ukraine.

While Britain has diverse gas supplies including connections with Norway and other European countries, in addition to three LNG import terminals, it still has some of the lowest levels of gas storage in Europe at 12 days on average and 7.5 peak winter days. Germany averages 89 days of stored gas, with France at 103 days and the Netherlands 123 days.

"I'm proud of the actions our team has taken over the last 18 months, including our decision to bring Rough back online, to underpin the UK's energy security," O'Shea said.

"However, we still have the lowest levels of energy storage of the world's major economies, with the ability to store fewer than eight days of peak winter demand - and this leaves us susceptible to shocks in international markets."

Centrica has said that Rough can balance the British energy system in the short term by storing gas when there is a surplus and then withdraw gas when needed during cold snaps and peak demand.

The long-term aim is to turn the Rough field into the largest long-range energy storage facility in Europe, capable of storing both natural gas and hydrogen, through investment of about 2 billion pounds ($2.53 billion) to quadruple its size.

($1 = 0.7903 pounds)

(Reporting by Marwa Rashad; Editing by David Goodman)