UK GAS-Prompt prices plummet as supply outstrips demand

* Gas market oversupplied by 30 mcm

* Friday's peak wind power output forecast at 6.6 GW

LONDON, Sept 8 (Reuters) - Prompt British gas prices fell as much as 10 percent on Thursday morning as stronger domestic supplies and an increase in imports from Norway led to a heavily oversupplied system.

Gas for immediate delivery fell 2.45 pence to 21.75 pence per therm by 0742 GMT, with gas for day-ahead delivery down 2.2 p at 22.25 p/therm.

Britain's gas system was oversupplied by 30 million cubic metres (mcm) per day, with demand forecast at 205.4 mcm and supply at 235.4 mcm, National Grid (LSE: NG.L - news) data showed.

Imports from Norway through the Langeled pipeline were about 27 mcm, up from 20 mcm the previous day.

The UMM Slal liquefied natural gas (LNG) tanker arrived in Britain on Thursday, with two more LNG tankers expected to bring additional supply over the next six days.

Domestic supply of gas was also strong. Flows through Bacton Seal were about 12 mcm after the gas terminal returned from an unplanned outage on Wednesday.

Meanwhile, forecasts for high output from Britain's wind farms led to reduced demand for gas from gas-fired power stations.

Wind power output in Britain was expected to peak at about 5.6 gigawatts (GW) on Thursday and 6.6 GW on Friday, up from levels below 2 GW earlier in the week, National Grid data showed.

Gas prices further out on the curve were little changed. The Winter 16 contract edged 0.05 pence higher to 38.25 pence/therm.

The Summer 17 contract gained 0.15 pence to 35.90 pence/therm.

In the Netherlands, the day-ahead gas price at the TTF hub was down 0.2 euros at 11 euros per megawatt-hour.

In the European carbon market, the benchmark Dec.16 contract slipped by 0.03 euros to 3.98 euros a tonne. (Reporting by Susanna Twidale; Editing by David Goodman)