UK GAS-Rising supply weakens prices, day-ahead power price jumps

June 23 (Reuters) - British prompt gas prices fell on Friday as flows from Norway increased amid expectations of higher output from domestic fields. * The British within-day gas price was down by 2 pence, or 5.33 percent, at 35.50 pence/therm at 0957 GMT. * The day-ahead gas price fell by 0.05 pence to 35.55 pence/therm. * Britain's gas system was undersupplied by 6.3 million cubic metres (mcm), with demand forecast at 165.5 mcm and flows at 159.2 mcm/day, National Grid (LSE: NG.L - news) data showed. * Norwegian flows through the Langeled pipeline rebounded to 45 mcm/day from 17 mcm in the previous session following the end of maintenance restricting output from the Kollsnes processing facility. * UK Continental Shelf production is expected to hit 121 mcm/day on Monday from about 106 mcm/day currently, according to data. * Britain's day-ahead power contract shot up to 60.50 pounds per megawatt hour (MWh), compared with the spot delivery contract which traded at 37.06 pounds per MWh, reflecting tight supply margins for Monday, traders said. * Power demand is forecast to be 1 gigawatt per hour higher on Monday due to reduced contributions from solar and wind, analysts said. * The July gas contract fell by 0.05 pence to 35.55 p/therm. * Day-ahead gas at the Dutch TTF hub was down by 0.18 euros at 15.05 euros/MWh. * The benchmark Dec (Shanghai: 600875.SS - news) -17 EU carbon contract fell by 0.05 euros to 4.91 euros a tonne. * Thomson Reuters (Dusseldorf: TOC.DU - news) analyst view: http://emea1.apps.cp.extranet.thomsonreuters.biz/cms/?pageid=united-kingdom-gas (Reporting by Oleg Vukmanovic; Editing by Mark Potter)