UK GAS-Prompt prices slip as supply outstrips high demand

* Gas system oversupplied by 16.5 mcm

* Norwegian supplies through Vesterled up 10 mcm

* Demand 11 pct above seasonal norm

May 19 (Reuters) - Prompt natural gas prices in Britain fell on Tuesday morning as strong supplies from Norway outpaced higher than normal demand for gas.

Prices for within-day delivery were at 44.1 pence per therm at 0811 GMT, down 1.1 pence since their last settlement. Prices for delivery on Wednesday were down 1.05 pence at 44.1 pence per therm.

With supply flows at about 214.6 million cubic metres (mcm) per day on Tuesday and demand expected to be about 198.1 mcm, the system was 16.5 mcm oversupplied, National Grid (LSE: NG.L - news) data showed.

Norwegian imports via the Langeled pipeline dropped to zero on Tuesday morning, the grid data showed, because of planned maintenance at Britain's Easington gas terminal.

However an increase in supplies from Norway through the Vesterled pipeline helped to offset the drop at Langeled.

"Receipts at UKCS (UK Continental Shelf) terminals are 13 mcm higher than yesterday this morning and nominated at 145 mcm for the day. The increase is due to Vesterled imports, which are 10 mcm/day higher this morning," analysts at Thomson Reuters Point Carbon said in a daily research note.

An outage at Britain's largest gas storage site Rough also meant there was less opportunity to inject gas into storage, adding to overall supply.

Injection capacity at Rough would be reduced by 12 mcm per day between May 18 and May 24, operator Centrica (LSE: CNA.L - news) said.

Tuesday's gas demand was around 11 percent above the seasonal norm of 178 mcm, the grid data showed.

"The last weather forecasts suggest temperatures slightly below the historical average the in northwestern Europe and UK up to the end of May, and this could sustain gas demand above its seasonal average," analysts at Italy's Banca IMI said in a research note on Monday.

In the Netherlands, the day-ahead gas price at the TTF hub was 0.18 euros lower at 20.52 euros per megawatt-hour (MWh).

The benchmark European Union carbon price dipped by 0.07 euros to 7.56 euros a tonne on ICE Futures Europe. (Reporting By Susanna Twidale; Editing by David Goodman)