* Langeled flows from Norway fall by 12 mcm/day
* Gas system undersupplied by 21 mcm/day
June 9 (Reuters) - British wholesale gas prices rose on Thursday as lower imports from Norway left the system undersupplied though the situation is expected to improve as Norway's biggest gas field has started to ramp up production.
Gas for immediate delivery rose 0.65 pence or 1.91 percent to 34.65 pence per therm by 0800 GMT, while day-ahead contract gained 0.25 pence or 0.74 percent to 34.25 pence/therm.
Demand on Thursday is forecast at around 176 million cubic metres (mcm), 4 mcm above the seasonal norm, and with supplies at 155 mcm/day, leaving the system 21 mcm undersupplied, according to National Grid (LSE: NG.L - news) data.
Norwegian gas flows through the Langeled pipeline, Britain's main subsea gas import route, fell by 12 mcm on Thursday morning as gas was rerouted to the Continent following the restart of flows at the Emden terminal in Germany.
Norway's Kollsnes gas processing plant and Troll, its biggest gas field, also started to raise production after annual maintenance, data from gas system operator Gassco showed.
Further along the curve, contract for gas delivery next-month fell by 0.55 pence to 33.25 pence/therm, while gas for winter was 0.9 pence down to 39.75 pence/therm.
In the Netherlands, the day-ahead gas price at the TTF hub fell 0.33 euro to 14.20 euro/MWh.
European front-year carbon allowances eased 0.07 euro to 6.04 euro a tonne.
While globally gas demand is expected to remain unchanged or even grow by 2040, it is expected to decline in Europe, Norway's Statoil said on Thursday in its long-term energy market outlook. (Reporting by Nerijus Adomaitis; editing by Jason Neely)