* Within-day price down 1.60 p at 28.50 p/therm
* Gas system oversupplied by 13 mcm/day
LONDON, Aug 24 (Reuters) - British wholesale gas prices fell on Wednesday morning as an increase in domestic output left the market oversupplied.
Gas for immediate delivery was down 1.60 pence at 28.50 pence per therm at 0901 GMT, while the day-ahead contract was 0.95 pence lower at 28.60 p/therm.
Britain's gas system was oversupplied by 13 million cubic metres (mcm) on Wednesday, with demand forecast at 172 mcm, broadly in line with seasonal norm, and supply at 185 mcm, National Grid (LSE: NG.L - news) data showed.
Nominations from the UK Continental Shelf (UKCS) rose by around 10 mcm from Tuesday, and more gas is expected to come from liquefied natural gas terminals, analysts at Thomson Reuters said.
"Prompt prices have opened the session lower, coming under pressure from the increase in UKCS supply together with downwards direction from further out," one UK-based trader said.
Maximum wind power output in Britain was expected to fall to 0.7 gigawatt (GW) on Thursday from 2.2 GW on Wednesday, National Grid data showed.
Higher-than-average temperatures are expected to continue until at least Sunday, before the weather becomes more unsettled but no major windy or cool spell is likely, said Thomson Reuters (Dusseldorf: TOC.DU - news) meterologist Georg Muller.
Further along the curve, the September contract eased by 0.87 pence to 29.15 p/therm, while gas for delivery this winter was 0.50 pence lower at 40.10 p/therm.
In the Dutch gas market, the day-ahead price at the TTF hub was 0.22 pence lower at 11.13 p/therm.
Norwegian exports to continental Europe were higher on Wednesday compared to the previous gas day but gas deliveries through from France were lower.
In the European carbon market, front-year allowances inched down by 0.01 euro to 4.70 euros a tonne. (Reporting by Nerijus Adomaitis; editing by Nina Chestney)