* Bearish prompt triggers sell-off in long-term market
* Prompt contracts defy rise in gas demand
* Rough storage helps balance system
LONDON, Jan 30 British gas for delivery this summer plunged to a fresh one-and-a-half-year low on Thursday as weakness in prompt prices translated into losses across the curve.
The summer 2014 gas contract shed 0.55 pence to 60.65 pence ($1) per therm on Thursday, the lowest since August 2012. The contract has posted a 4 percent loss in just one week.
Bearish momentum from the prompt, where contracts have been sliding due to mild weather, has moved over to the long-term market and triggered a sell-off of seasonal contracts.
British prompt gas prices also posted steep losses on Thursday despite a rise in demand.
Gas for within-day delivery fell 0.95 pence to 62.25 pence per therm, while day-ahead prices traded at 62.50 pence, down 0.85 pence day on day.
"Gas falls further, shrugging off higher demand as supply confidence persists due to mild temperature forecasts," one gas trader said.
Prompt gas prices were at levels last seen in November, an atypical situation for January, when demand usually rises.
The gas system balanced in the late morning after opening short as Britain's gas storage sites helped make up for the shortfall.
The Rough storage facility, Britain's largest, was flowing at a rate of around 45 million cubic metres per day (mcm/d) on Thursday morning, National Grid data showed.
Weather forecasts for the remainder of the week and the week ahead pointed to continued mild temperatures.
The contract for gas deliveries on weekdays next week fell 0.80 pence to 62.10 pence per therm.
Prices in Britain's power market also fell on Thursday as day-ahead wind power production levels were forecast to reach highs of around 6 gigawatts (GW).
The day-ahead baseload contract slipped 2.45 pounds to 47 pounds per megawatt-hour.
EDF Energy's 550-MW Dungeness B22 nuclear reactor is scheduled to go offline on Friday for a planned outage. ($1 = 0.6041 British pounds) (Reporting by Karolin Schaps; editing by Jane Baird)