* Gas system oversupplied by 24 mcm/day
* Weather seen to become significantly warmer
* Rough outage expected to last until May 9-11
May 4 (Reuters) - British spot gas prices fell on Wednesday as rising temperatures were set to reduce demand already curbed by outage at the country's main gas storage site.
Gas for immediate delivery fell 0.75 pence, or 2.6 percent, to 28.15 pence per therm by 0830 GMT, while the day-ahead contract lost 0.75 pence, or 2.58 percent, to 28.35 pence/therm.
Prompt prices had fallen to levels seen last time on April 20 before rallying to an almost four-month high of 35 pence/therm on April 26 after unseasonably cold weather increased demand.
The latest forecasts showed weather in Britain turning significantly warmer from Wednesday, with temperatures seen rising to over 20 degrees in the south by the weekend from a high of 18 degrees on Thursday.
Meanwhile, injection capacity at the country's biggest gas storage site, Rough, has been reduced by 26 mcm/day from April 27, and the outage is not expected to end before May 9.
In addition injection capacity at Aldbrough gas storage was cut on Wednesday morning due to an outage expected to last until midday.
"Given the likely decline in weather related demand, the lack of injection capacity will weigh on NBP (Shanghai: 601018.SS - news) (British gas hub) prices this week," London-based Energy Aspects said in a note.
Gas is traditionally injected into storage during the warmer summer months when demand is weak.
Pressure on prices was offset by gas imports from Norway falling 16 mcm to 78 mcm/day due to lower deliveries via the Langeled and the Vesterled pipelines, data from gas system operator Gassco showed.
With (Other OTC: WWTH - news) gas supply flows at about 243 million cubic metres (mcm) and demand expected to be about 219 mcm, Britain's gas system was 24 mcm oversupplied, National Grid (LSE: NG.L - news) data showed.
Further along the curve, the contract for gas delivery next-quarter was at 28.30 pence/therm, 0.15 pence or 0.53 percent down.
The contract for gas delivery next winter was at 34.90 pence/therm, down 0.1 pence, or 0.29 percent.
In the Netherlands, the day-ahead gas price at the TTF hub fell 0.11 euro/MWh, or 0.9 percent, to 12.07 euro/MWh.
In Europe's carbon market, the front-year EU allowance (EUA) eased 0.04 euro, or 0.67 percent, to 5.95 euro a tonne. (Reporting by Nerijus Adomaitis in Oslo, editing by Louise Heavens)