* UK gas system 15 mcm undersupplied * Brent crude rises back above $100/barrel By Henning Gloystein LONDON, June 11 (Reuters) - British prompt gas prices rose on Monday morning as the system was almost 15 million cubic metres (mcm) undersupplied and forward prices were lifted on the back of Brent crude prices rising back above $100 per barrel. Gas prices for delivery next day were trading at 55.90 pence per therm at 0945 BST (0845 GMT), up 0.95 pence from Friday morning, and prices for within-day delivery were up 0.75 pence to 55.85 pence a therm. Analysts said that the rises were the result of an undersupplied market and rising consumption expected this week. "Consumption is forecasted up for day-ahead, which is bullish for spot gas prices, and the other bullish elements are increased oil prices and the short system this morning," analysts at Point Carbon said. Daily gas demand was expected to be 217.7 mcm on Monday, around 15 percent below the seasonal norm, according to National Grid, but still around 15 mcm above expected flows. Power prices for baseload (24 hours) delivery on Tuesday were at 43.80 pounds per megawatt-hour (MWh), up 1.3 pounds per MWh. Further out on the curve, gas prices for delivery next winter were up slightly to 65.75 pence per therm, fuelled by front-month Brent crude rising back above $100 per barrel. "Brent oil will edge up into a range of $103.30-$104.01 per barrel as rebound from the June 4 low of $95.63 has extended," Reuters' technical analyst Wang Tao said.

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