UK GAS-British prompt prices plunge on oversupply

* Within-day gas price falls 10 pct

* Langeled flows rise as demand falls

* Two new LNG cargoes heading to Britain

Aug 15 (Reuters) - British prompt gas prices plunged on Monday morning as rising deliveries from Norway, lower exports to Belgium and weaker demand left the system heavily oversupplied.

Gas for immediate delivery was down 3.25 pence or 10.24 percent, to 28.50 pence per therm by 0842 GMT, while the day-ahead contract lost 2.35 pence or 7.5 percent, to 29.00 pence/therm.

The UK gas system was about 29 million cubic metres (mcm) oversupplied, based on opening and expected end-of-the-day linepack gas volumes, National Grid (LSE: NG.L - news) data showed.

"Total (LSE: 524773.L - news) demand has plummeted to 134 mcm, down 20 percent from the seasonal normal level, in line with near zero injections into storage and just 6.6 mcm of exports through IUK," one UK-based trader said.

Norwegian gas deliveries through the Langeled pipeline, Britain's main subsea gas import route, rose to 25 mcm/day on Monday morning from 17 mcm/day on Friday.

"Strong Langeled flows (from Norway) over the weekend in a period of almost full storage have really made the market nervous," said Dimitrios Saratzis, a gas market analyst at Thomson Reuters (Dusseldorf: TOC.DU - news) .

Gas flows from the UK continental shelf were nominated at 112 mcm/day, up from 105 mcm/day on Friday, while exports to Belgium via the Interconnector pipeline were down to 8 mcm/day from 15 mcm/day on Friday.

One LNG tanker delivered a cargo to Britain over the weekend, while another two are expected to arrive around Aug. 20-25.

Dutch day-ahead gas prices at the TTF hub also took a dive, falling 0.63 euro or 5.38 percent to 11.07 euros per megawatt-hour (MWh).

Further along the curve, the British contract for gas delivery next-month was at 30.45 pence/therm, down 1.3 pence or 4.1 percent, while gas for next winter was unchanged at 41.40 pence per therm.

Longer-term contracts got some support from stronger oil prices, which rose to fresh highs for the month as speculation intensified about potential producer action to support prices in an oversupplied market.

In the European carbon market, front-year allowances were unchanged to 4.90 euros a tonne. (Reporting by Nerijus Adomaitis; editing by Jason Neely)