* Day-ahead price at December 2011 level
* Summer gas price at record low
* Demand over 30 mcm below the seasonal norm
LONDON, March 31 (Reuters) - British gas prices slumped to new lows on Monday as warmer weather reduced demand for gas for heating systems.
Gas prices for next-day delivery were trading at levels last seen over two years ago, while summer gas prices slumped to record lows.
Gas for delivery on Tuesday was trading at 50.50 pence a therm at 0836 GMT, down 1.35 pence or 2.60 percent from Friday's close. The last time the contract was at this level was in December 2011.
Further along the curve, the price of gas for delivery this summer was trading 1.08 pence or 2 percent lower at 52.05 pence per therm, having touched a record low of 51.60 pence in earlier trade.
A mild winter has already left gas inventories unusually full for the time of year and mild conditions look set to continue, further reducing demand for gas, traders said.
"Consumption is significantly lower compared to last week on milder weather," said analysts at Thomson Reuters Point Carbon.
Demand for gas was forecast to be around 227 million cubic metres (mcm) on Monday, over 30 mcm below the seasonal norm of 258 mcm, according to the National Grid.
Gas supply was seen at around 234 mcm/day, meaning the British gas system was oversupplied by around 7 mcm of gas.
Temperatures in some parts of Britain are seen reaching a high of 18 degrees Celsius by the middle of this week, according to Britain's Met Office.
This compares to Britain's average yearly temperature in March of around 10 degrees C.
German power prices for next year also continued their downtrend on Monday, driven by weak demand and oversupply in the market. (Reporting by Nina Chestney; editing by Keiron Henderson)