UK GAS-Weekend prices gain on tighter supply outlook

* Gas system undersupplied by 9 mcm/day

* Oil prices retreat below $50 a barrel

* Norway, UK maintenance to tighten supply

May 27 (Reuters) - British within day prices fell on Friday on warmer weather that dented demand, while prices for the weekend and next month rose on expected tightening of supply due to planned maintenance.

Gas for immediate delivery fell 0.4 pence or 1.25 percent, to 31.70 pence per therm by around 0850 GMT, while prices for the weekend gained 0.65 pence or 2.16 percent to 30.75 pence/therm.

Within-day prices fell on warmer weather, while exports to Belgium fell and supply from liquefied natural gas (LNG) terminals remained healthy.

Maximum temperatures in the south of the country are forecast to rise to 21 degrees Celsius, up from 20 degrees on Friday, UK's Met Office said.

Gas imports from Norway also gained by 6 mcm to 32 mcm/day after an outage at Norway's biggest gas field Troll ended, while production capacity was still reduced at Kvitbejoern and Visund fields.

Total (LSE: 524773.L - news) supply was seen at 193 million cubic metres (mcm) per day on Friday versus demand at 202 mcm, leaving Britain's gas system undersupplied by 9 mcm, National Grid (LSE: NG.L - news) data showed.

However, planned maintenance in Norway and Britain is expected to tighten supply.

Norway's key gas processing plant Kollsnes starts ramping down output on Friday, and will be completely shut for annual maintenance along with the country's biggest gas field, Troll, on Saturday.

As a result, gas flows through the Langeled pipeline, Britain's main subsea gas import route, could fall to 8 mcm/day, analysts at Thomson Reuters Point Carbon said.

Flows from the UK continental shelf are also expected to fall in the beginning of June due to maintenance at the Teesside gas processing plant and at St Fergus terminal.

The contract for gas delivery next month was trading at 30.75 pence/therm, 0.25 pence up, while gas for next-winter delivery was steady at 35.90 pence/therm.

Brent oil futures slipped below $50 a barrel on Friday, capping gains in longer-term gas contracts.

The Dutch day-ahead gas price at the TTF hub rose 0.05 euro/MWh or 0.37 percent, at 13.65 euro/MWh.

In the European carbon market, front-year allowances eased 0.08 euro or 1.33 percent to 5.95 euro a tonne. (Reporting by Nerijus Adomaitis in Oslo; editing by Susan Thomas)