UK GAS-Within-day prices fall on oversupplied system

* System oversupplied by 21 mcm

* Norway to start heavy gas maintenance next week

Aug 26 (Reuters) - British within-day gas prices fell on Friday due to an oversupplied system, while those for Monday rose as planned maintenance is expected to reduce imports from Norway.

Gas for immediate delivery lost 0.55 pence or 1.9 percent to 27.90 pence per therm by 0948 GMT, while the contract for delivery next Monday gained 0.05 pence to 28.30 pence/therm.

Britain's gas system was oversupplied by around 21 million cubic metres (mcm) with demand forecast to be 167 mcm and supply at 188 mcm/day, according to National Grid (LSE: NG.L - news) data.

An outage at Norway's Nyhamna gas-processing plant ended on Friday, supporting flows, but those were expected to drop next week due to extensive maintenance.

"We are now entering a period with heavy maintenance on Norwegian processing plants," analysts at Thomson Reuters (Dusseldorf: TOC.DU - news) wrote.

The Kaarstoe gas-processing plant will be completely shut from Saturday until Sept. 16, while production capacity at the Kollsnes plant will be cut by 70 mcm/day on Sept. 2-7.

Flows via the 72 mcm/day capacity pipeline will be completely shut for one day next Tuesday due to testing at the Easington terminal.

A reduction in Norwegian gas flows could be offset by higher deliveries of liquefied natural gas (LNG).

Four new tankers are likely heading to Britain, while one tanker was confirmed to deliver gas on Aug. 30. If other arrivals are confirmed, supply from LNG terminals could increase to 30 mcm/day from the current 20 mcm/day, the analysts said.

LNG imports could rise as demand from importers in the Middle East and Northeast Asia falls with the end of the peak cooling season, London-based consultancy Energy Aspects said.

With UK gas stocks nearly full owing to an outage at Britain's Rough storage site, more LNG could result in higher exports to Belgium, it added.

Further along the curve, prices for September eased by 0.08 pence to 29.05 pence/therm, while the next-winter contract was broadly steady at 40.10 pence/therm.

Dutch day-ahead gas prices at the TTF hub held steady at 11.10 euros per megawatt-hour.

European front-year carbon allowances firmed by 0.02 euro to 4.73 euros a tonne. (Reporting by Nerijus Adomaitis; Editing by Dale Hudson)