UK GAS-Prices firm, system near balanced

* System oversupplied by 5 mcm/day

* Temperatures forecast to stay below seasonal norm

LONDON, May 28 (Reuters) - British prompt gas prices firmed on Thursday, supported by a near-balanced system and forecasts for temperatures to remain below seasonal norms in the coming days.

Gas for within-day delivery was up 0.1 percent at 42.90 pence per therm at 0750 GMT, while gas for day-ahead delivery was up 0.2 percent at 42.95 pence.

National Grid (LSE: NG.L - news) data showed supply was forecast at 208 million cubic metres (mcm) on Thursday, while demand was seen at 203 mcm, showing the system was oversupplied by 5 mcm.

Meteorologists forecast temperatures holding below normal levels for the remainder of the week before rising next week.

Norwegian gas system operator Gassco revised the details of an unplanned outage at Kvitebjørn on Thursday, with flows down 16.1 mcm.

Thomson Reuters Point Carbon analysts said Norway's Langeled pipeline is nominated at 8 mcm, which is in line with yesterday and below the likely level if the Kvitebjorn field was operating at full capacity.

"An outage on that field is expected to reduce capacity tomorrow by 5 mcm/d, less than today and the reason we indicate higher Langeled flows tomorrow," analysts added.

Shipments of LNG continued at a steady pace, with three tankers expected to arrive at the UK's South Hook LNG terminal in the coming week.

Gas for delivery in the third quarter was up 0.3 percent at 41.92 pence per therm.

In the Netherlands, day-ahead gas at the TTF hub was down 0.5 percent at 20.60 euros per megawatt-hour.

The benchmark European Union carbon price was trading 0.1 percent higher at 7.22 euros a tonne on ICE Futures Europe. (Reporting by Sarah McFarlane; Editing by David Goodman)