LONDON, June 13 (Reuters) - Prompt gas prices in Britain fell sharply on Tuesday as a major gas pipeline with mainland Europe prepared to shut, trapping surplus gas in the UK until June 29.
* British within-day gas price was down by 1.40 pence at 33.70 pence/therm at 1006 GMT.
* Day-ahead gas fell 2.65 pence or 7.49 percent to 32.75 pence per therm in response to the shutdown of the Britain-Belgium gas link due on Wednesday, coupled with lower forecast demand at that time.
* The pipeline shutdown for annual maintenance until June 29 is the main bearish driver because it removes a key outlet for Britain to export surplus gas, traders and analysts said.
* Despite falling prices, the gas system is undersupplied by 9.9 million cubic metres (mcm), with demand forecast at 190.6 mcm and flows at 180.7 mcm/day, National Grid (LSE: NG.L - news) data showed.
* Falling deliveries from Norway through the Langeled pipeline to 18 mcm/day from around 34 mcm on Monday left the system facing shortfalls.
* One trader said low gas prices would likely fuel demand by gas-fired power plants during the weekend. Higher forecast wind farm output on Thursday and Friday could constrain gas-for-power consumption, he added.
* July gas down 0.47 pence at 35.28 p/therm.
* Day-ahead gas at the Dutch TTF hub down by 0.18 euro at 14.95 euros per megawatt hour.
* Benchmark Dec-17 EU carbon contract down by 0.05 euro at 4.97 euros per 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 David Evans)