LONDON—Britain is set to lose its largest natural gas storage site, increasing the country's reliance on imported energy, after British Gas owner Centrica said it would close its aging Rough facility.

Wholesale gas prices will become more volatile and more vulnerable to price spikes, analysts and traders said.

Britain already imports about half of its gas from Norway, continental Europe and from Qatar in the form of LNG.

This figure is expected to rise as its own supplies from the UK Continental Shelf decline.

And if Britain cannot call on stored reserves when demand rises in winter, it will need to import even more. Rough covered one-tenth of Britain’s peak winter demand.

“The loss of Rough will create uncertainty, volatility and leave (Britain) exposed,” said Wayne Bryan, an analyst at consultancy Alfa Energy.

“If we experience a two-to-three week cold snap, the loss of Rough will see us reliant on imports, namely LNG,” Bryan said.

Storage provides security and flexibility of supply. Gas injected in the summer at times of low demand and low prices is there to meet demand when consumption rises in the winter.

With the closure of Rough, Britain loses about 70% of its storage capacity.

“The market will be more exposed to international price fluctuations as more imports will be required to balance Britain’s gas market when cold,” said Katrina Oldham, an energy trader at Inenco.

Reflecting the British market’s vulnerability to imports, prices for July gas jumped earlier this month after doubts were raised about delivery of two Qatari LNG cargoes.

The two tankers eventually declared they would come to Britain, but via a different route, sending prices back down.