* UK gas demand falls 19 pct below norms * At least three LNG tanker due in UK in next fortnight LONDON Aug 1 (Reuters) - British prompt gas prices plunged on Monday as the market increased supply in anticipation of lower flows due to further planned maintenance, traders said. The gas market was oversupplied by around 10 million cubic metres (mcm) on Monday, after supply from Norway via the Langeled pipeline and liquefied natural gas (LNG) terminals rose to make up for maintenance supply losses. "There is plenty of supply maintenance this week, so the market thought there might be a struggle to balance. However flows from Langeled and demand side weakness make the system nice and long," one UK gas trader said. Demand for British gas fell 19 percent below seasonal norms on warm weather across the UK and demand for British gas from Europe was weak due to summer holidays. UK gas for day-ahead delivery slipped 1.10 pence to 51.75 pence per therm, while Monday gas traded at 51.90 pence, down 1.10 pence from the previous session. Incoming front-month September edged lower in line with the prompt, shedding 0.75 pence compared with the previous session to 51.50 pence. Three more LNG tankers are expected to arrive in Britain over the next two weeks, further providing a comfortable outlook for LNG supply over the coming weeks.

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