UK GAS-Prompt prices slide on warmer weather forecasts

* Gas system undersupplied by 6.1 mcm

* Temperatures over 20 degrees C expected by Thursday

* Three LNG cargoes docked or heading to Britain this week

LONDON, June 1(Reuters) - Prompt natural gas prices in Britain fell on Monday morning as forecasts for warmer weather and expectations of supply from liquefied natural gas (LNG) tankers weighed on prices.

Prices for within-day delivery were 43.70 pence per therm at 0930 GMT, down 0.10 pence since their last settlement. Prices for delivery on Tuesday were down 0.15 pence at 43.65 pence per therm.

With supply flows at about 191.6 million cubic metres (mcm) per day on Monday and demand expected to be about 197.7 mcm, the system was 6.1 mcm undersupplied, National Grid (LSE: NG.L - news) data showed.

"Despite strained supplies intra-day, prompt prices have softened in line with a forecast increase in average temperatures for the coming week," said Marcel Boonaert, head gas trader at Wingas UK.

Britain's Met Office forecasts temperatures in part of the country will rise to more than 20 degrees Celsius by Thursday up from an average of around 10-14 degrees on Monday.

Boonaert also said expectations of gas supplies from LNG tankers coming to the country put downwards pressure on prices.

"Confirmation of an additional cargo into South Hook within the week also improves supply outlooks for June despite the turn down in flows today," he said.

Three LNG cargoes are either docked or heading to Britain over the next week.

Gas prices further along the curve edged lower, with the Winter 2015 contract down 0.20 pence at 47.90 pence per therm.

In the Netherlands, the day-ahead gas price at the TTF hub was 0.05 euros lower at 20.55 euros per megawatt-hour (MWh).

The benchmark European Union carbon price dipped by 0.08 euros to 7.28 euros a tonne on ICE Futures Europe. (Reporting by Susanna Twidale; editing by Susan Thomas)