* Gas system undersupplied by 11 mcm
* Oil gains on plans for global output freeze
March 17 (Reuters) - British long-term gas prices gained on Thursday morning, lifted by stronger oil, while prompt prices were broadly steady as the system's balance flipped from surplus to deficit.
Gas for immediate delivery fell 0.6 pence or 1.98 percent, to 29.70 pence per therm by 1008 GMT, while the day-ahead contract gained 0.03 pence or 0.1 percent to 29.98 pence/therm.
With (Other OTC: WWTH - news) supply flows at about 276 million cubic metres (mcm) and demand expected to be 287 mcm, Britain's gas system was 11 mcm undersupplied, National Grid (LSE: NG.L - news) data showed.
The balance flipped from a 8 mcm surplus earlier on Thursday.
"I find it difficult to explain what has caused such a shift. Maybe we are going to see some outages later today which haven't been reported yet," a UK-based trader said.
However, supply still looked quite comfortable with deliveries from liquefied natural gas (LNG) terminals rising, and flows from Norway remaining strong, he added.
LNG supply was nominated at 38 mcm/d, up from 31 mcm on Wednesday.
Norway's production capacity was impacted by two field outages, but flows to Britain via the Langeled pipeline edged higher by 3 mcm to 71 mcm/day.
"As we head into Easter, a gradual transition to more unsettled and slightly colder weather looks the more likely trend, with stronger winds and outbreaks of rain affecting the north and northeast," UK Met Office said.
Further along the curve, gas for delivery next season was at 27.50 pence/therm, up 0.27 pence.
"We see some stronger sentiment from oil prices," the trader said.
Brent crude oil rose 52 cents to $40.85 a barrel, after ending the previous session 4.1 percent higher as the world's biggest suppliers firmed up plans to meet to discuss an output freeze.
Gas for delivery next winter was at 33.35 pence/therm, up 0.36 pence.
The Dutch day-ahead gas price at the TTF hub fell by 0.1 euro/megawatt hour to 12.30 euro/MWh.
In the European carbon market, front-year EU allowances eased by 0.03 euro to 4.94 euro a tonne. (Reporting by Nerijus Adamaitis)