Shutdown at UK's Rough gas storage site could last into 2017

* Tests on additional wells needed

* Completion expected March/April 2017

* Some wells could return to service by Nov. 1, 2016 (Adds detail, comment, prices)

By Nina Chestney

LONDON, July 15 (Reuters) - The shutdown of Britain's largest gas storage facility, Rough, was extended into March or April next year on Friday, increasing the possibility Britain could go into winter with record low inventories and sending winter gas prices soaring.

In March last year, operator Centrica Storage Limited (CSL (Other OTC: CMXHF - news) ) imposed reductions on how much gas could be stored at Rough as a safety precaution after identifying potential issues with well integrity.

Last month, it said testing was expected to last for 42 days, or until Aug. 3, meaning injection and withdrawal of gas into and from the site would not be possible until then.

CSL said on Friday it had completed testing on the affected well earlier than thought and has plugged it, but has identified potential uncertainties in other untested wells.

"We estimate completion in March to April 2017. In the meantime because of the uncertainty as a prudent and safe operator CSL cannot inject or withdraw gas from Rough," the firm said in a statement.

UK natural gas prices for winter delivery jumped by around 8 percent to 47.00 pence per therm.

"This is obviously bullish and the winter 2016 price is getting a beating on this news," said Dorian Lucas, gas trader at Inenco.

"If we get a mild winter like the last few then things shouldn't be that bad, but it will still be bullish. This could also filter through to higher demand on the Continent and maybe the Dutch could raise production from Groningen. Obviously LNG will play a big role in meeting UK demand," he added.

However, CSL added it was examining whether some wells could return to service for the winter 2016/17 withdrawal season.

"CSL currently anticipates that at least 4 wells will return to service for withdrawal operations by Nov. 1, 2016," it said.

Britain depends in large part on stored reserves to manage winter demand spikes while domestic stockpiles also help ensure security of energy supplies.

Reserves are typically replenished during summer months when demand and prices are low.

National Grid (LSE: NG.L - news) said earlier this month Britain would have sufficient means of meeting demand even if Rough is not fully filled this year.

(Additional reporting by Oleg Vukmanovic; editing by David Evans)