UK GAS-Prompt firms on tight supply as outages' end weighs on curve

* Spot gas down a penny to 30.05 p/therm

* Three major outages set to end by Sunday

* Rising supply confidence weighs on curve prices

Aug 18 (Reuters) - British gas prices rose on Thursday due to supply shortfalls but with heavy maintenance drawing to an end on key infrastructure analysts expect supplies to improve by next week.

Gas for immediate delivery rose by 1.00 pence to 30.05 pence per therm by 0835 GMT, while the day-ahead contract was up by 0.45 pence at 30 pence/therm.

With (Other OTC: WWTH - news) forecast supply flows at 166.3 million cubic metres (mcm) and demand expected at 171.7 mcm/day, Britain's gas system was 5.4 mcm undersupplied, National Grid (LSE: NG.L - news) data showed.

Gas deliveries from Britain to Belgium are estimated to hit 40 mcm/day - up three mcm/day from Wednesday's levels - while flows from LNG terminals fell by 4 mcm to 23 mcm/day.

By Sunday a major North Sea gas field, pipeline system and gas import terminal should resume operations following a period of maintenance which should weigh on prices as supplies rise, analysts at Thomson Reuters (Dusseldorf: TOC.DU - news) said.

The Britannia gas field is to resume output at 1000 GMT on Friday, in time boosting flows into the St. Fergus Mobil terminal by as much as 10 mcm/day, although analysts expect flows into the terminal to rise by just 2 mcm/day on Friday.

On Saturday, outages affecting the Bacton SEAL gas pipeline system as well as the SAGE pipeline and receiving terminal are due to end.

Rising supply confidence helped push down some gas prices further along the curve. The month-ahead September contract was down by 0.25 pence at 30.95 p/therm.

Along the curve, the benchmark winter 2016 contract rose in tandem with crude oil prices to trade 0.10 higher at 43 pence per therm.

Oil prices rose on Thursday, with Brent topping $50 a barrel for the first time in six weeks, as traders continued to talk up the potential for an agreement on freezing output levels at a meeting of OPEC and non-OPEC producers next month.

In the Netherlands, the day-ahead gas price at the TTF hub fell 0.03 euro to 11.57 euros per megawatt-hour.

In Europe's carbon market, front-year allowances rose 0.07 euro to 4.56 euros per tonne. (Reporting by Oleg Vukmanovic in Milan; editing by Jason Neely)