UK GAS-Prompt gas prices rise as demand outstrips supply

* Market undersupplied by 9.9 mcm

* Britannia field outage curbs domestic supply

LONDON, March 16 (Reuters) - British spot gas prices rose on Wednesday as maintenance at the Britannia gas field curbed gas supplies while forecasts for low wind power output led to strong demand from gas-fired generators.

Gas for within-day delivery gained 0.20 pence to 30.70 pence per therm at 0913 GMT.

Gas prices for day-ahead delivery rose 0.35 pence to 30.50 pence per therm.

With (Other OTC: WWTH - news) supply flows at about 286.2 million cubic metres (mcm) and demand expected to be 296.1 mcm, Britain's gas system was 9.9 mcm undersupplied, National Grid (LSE: NG.L - news) data showed.

Domestic supplies dipped on Wednesday as maintenance at the Britannia field cut supply through the St Fergus Mobil gas terminal to around 13 mcm/day from levels near 20 mcm/day before the outage, National Grid data showed.

Meanwhile Norwegian gas capacity will fall by 8.9 million cubic meters (mcm) on Wednesday due to an unexpected outage, gas system operator Gassco said in a market message.

Imports to Britain from Norway via the Langeled pipeline were stable on Wednesday morning around 70 mcm/day.

Traders said forecasts for weak output from Britain's wind farms had also helped to ramp up demand for gas from power plants.

Peak wind power generation was forecast to reach 2.3 gigawatts GW) of electricity on Wednesday falling to 1.5 GW on Thursday, National Grid data showed, well below the total capacity around 9 GW.

Further out on the curve, the Winter '16 contract rose 0.08 pence to 33.08 pence per therm.

In the Netherlands, day-ahead gas at the TTF hub rose 0.1 euros to 12.35 euros per megawatt-hour.

In Europe's carbon market, the front-year EU Allowance price edged up 0.06 euros to 4.93 euros a tonne. (Reporting By Susanna Twidale)