UK GAS-British prompt prices surge on Norwegian supply cuts

* UK gas system undersupplied by 31.7 mcm

* Output from Norway's Troll field down 117 mcm

LONDON, Jan 14 (Reuters) - British prompt natural gas prices rose by more than 5 percent on Wednesday morning as a large outage at Norway's biggest gas field slashed export flows from the UK's top external supplier.

Prices for within-day delivery were trading at 49.80 pence per therm at 0855 GMT, up 2.6 pence or 5.5 percent since their last settlement. Prices for delivery on Thursday were up 1.5 pence at 49.10 pence per therm.

Production at Norway's Troll gas field will be reduced by 117 million cubic metres (mcm) per day on Wednesday, and the outage could last for one to two days, Norwegian gas system operator Gassco said on its website.

As a result gas flows via the Langeled pipeline, Britain's main subsea gas import route, were down below 20 mcm/day on Wednesday morning compared to its full transportation capacity of around 70 mcm/day.

With supply flows to Britain at about 265.9 mcm per day on Wednesday and demand expected to be about 297.6 mcm, the system was 31.7 mcm undersupplied, National Grid (LSE: NG.L - news) data showed.

Troll's operator Statoil (Xetra: 675213 - news) said it was preparing to restart gas production but had no estimate yet on when exports would start to flow.

Further along the curve, gas for February delivery was 0.50 pence higher at 48.90 pence per therm while the Summer 15 contract rose 0.15 pence to 44.40 pence per therm.

In the Netherlands, the day-ahead gas price at the TTF hub was 0.35 euro higher at 20.50 euros per megawatt-hour.

The benchmark European Union carbon price dipped 0.04 euro to 7.34 euros a tonne on ICE Futures Europe. (Reporting by Susanna Twidale; Editing by Sam Wilkin)