UK GAS-Prompt prices firm as supplies from Norway drop

* UK gas system undersupplied by 3.5 mcm

* Outage cuts Norway's gas output by 20 mcm

LONDON, Jan 9 (Reuters) - Prompt natural gas prices in Britain firmed on Friday as an unplanned outage cut flows from Norway, leaving the system undersupplied.

Prices for within-day delivery were at 46.15 pence per therm by 0905 GMT, up 0.60 pence since their last settlement. Prices for delivery on Monday rose 0.80 pence to 46.00 pence per therm.

With supply flows at about 240.7 million cubic metres per day (mcm/d) on Friday and demand expected to be about 244.2 mcm, the system was 3.5 mcm undersupplied, National Grid (LSE: NG.L - news) data showed.

Norway's gas output will be down by 20 mcm on Friday and 23 mcm on Saturday due to an unexpected outage at Statoil (Xetra: 675213 - news) 's Kvitebjoern field, the country's gas system operator Gassco said.

Norwegian imports to Britain via the Langeled pipeline were around 50 mcm/d on Friday morning, down from around 60 mcm/d on Thursday, National Grid data showed.

Langeled is Britain's main sub-sea gas import route and has a capacity of around 70 mcm/d.

Further along the curve, the February contract was 0.55 pence higher at 46.85 pence per therm while the summer '15 contract rose 0.80 pence to 44 pence per therm.

In the Netherlands, the day-ahead gas price at the TTF hub was flat at 19.40 euros per megawatt-hour.

Analysts at consultancy Energy Aspects forecast Dutch gas demand would rise 0.9 billion cubic metres or 8.5 percent in the first quarter of 2015 compared with a year earlier, with a mild start to January and some forecasts of another mild winter.

The benchmark European Union carbon price dipped 0.08 euro to 6.78 euros a tonne on ICE Futures Europe. (Reporting by Susanna Twidale; Editing by Dale Hudson)