UK GAS-Prompt prices rise in undersupplied market

LONDON, July 3 (Reuters) - Prompt British wholesale gas rose on Monday morning in an undersupplied market, as forecasts of weak output from the country's wind farms led to an increase in demand from gas-fired power plants. * British within-day gas price up by 1.15 pence to 37.60 pence/therm at 0840 GMT. * Day-ahead gas price up by 1.45 pence to 37.70 p/therm. * British gas system undersupplied by 28.2 million cubic metres (mcm), with demand forecast at 203 mcm and flows at 174.8 mcm/day, National Grid (LSE: NG.L - news) data showed. * "Lower wind means gas-to-power demand is up about 4 mcm from Friday," a gas trader said. He added that exports from Britain to Belgium via the IUK interconnector were also up, increasing the undersupply in the market. * Peak wind power generation is forecast at 4.7 gigawatts (GW) on Monday but is expected to drop to below 1 GW on Tuesday, National Grid data showed. * Weak wind output, and the closure of two of Britain's nuclear reactors for maintenance, also led to an increase in prompt electricity prices. * British wholesale day-ahead electricity price up by 8.72 pounds to 52.00 pounds/megawatt hour (MWh). * The Hartlepool 2 reactor went offline over the weekend for maintenance, operator EDF Energy said in a market update, while the Dungeness B-21 reactor is also offline. * August gas contract up by 0.75 pence to 36.95 p/therm. * Day-ahead gas price at the Dutch TTF hub down by 0.22 euro to 15.13 euros per megawatt hour. * Benchmark Dec-17 EU carbon contract down by 0.01 euro to 5.09 euros per tonne. (Reporting by Susanna Twidale; editing by Nina Chestney)