* Drop in Bacton Shell flows leaves system short * Winter gas firms, could rise further - trader * Dungeness B22 nuclear reactor in unplanned outage LONDON, Sept 6 (Reuters) - British prompt gas prices rose to a four-month high on Thursday as maintenance in the North Sea and a lack of liquefied natural gas (LNG) supplies led to tightness in the system. The day-ahead contract touched 60.75 pence per therm in early trade, its loftiest level since May 4 and up 0.75 pence day on day. Prices had eased to 60.40 pence by 1050 GMT but remained 0.40 pence up on Wednesday's settlement. "The system is short because of North Sea maintenance and there's a lack of LNG," one gas trader at a utility said. The UK gas system was around 9 million cubic metres per day (mcm/d) short, National Grid data showed, after flows from Shell's Bacton gas terminal dropped to zero. Shell declined to comment, but analysts at Point Carbon, a Thomson Reuters company, said the drop in flows could indicate an early start to scheduled maintenance. Lower LNG imports were also adding to the tightness in the system, traders said, but two deliveries were expected at British terminals on Thursday and Friday. "Total imports (including LNG send out) are currently 59 mcm/d, down from yesterday's average of 67 mcm/d. The drop is mainly due to lower BBL flows," Point Carbon analysts said. Further out on the curve prices are likely to be dictated by economic news as energy market investors await the outcome of Thursday's European Central Bank meeting. Front-month Brent crude briefly rose above $114 per barrel on hopes that policy makers will announce a new programme of bond purchases to help ease the eurozone debt crisis. Winter 2012 gas prices ticked up 0.25 pence to 66.85 pence, near a four-month high, and the gas trader warned winter prices could firm ahead of the contract's expiry on October 1. "Overall most people are bearish on the outturn (of the winter prices), but I don't think there's enough risk premium in the price for now," he said. In Britain's power market the day-ahead baseload contact rose 27 pence to 43.08 pounds per megawatt-hour (MWh) on the back of firmer gas and an unplanned nuclear outage. EDF's 550 MW Dungeness B22 nuclear reactor ceased electricity output to the transmission network late on Wednesday for an unplanned outage, reducing supply just one day after it had restarted following maintenance work. Winter 2012 baseload power prices nudged up 15 pence to 49.90 pounds. (Reporting by Susanna Twidale; Editing by Karolin Schaps and Alison Birrane)

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