UK GAS-Prices fall on mild weather, long system

* UK gas system opens 8 mcm long

* Mild weather curbs demand, flows steady

* Norwegian output to drop 24 mcm/day on April 19-21

LONDON, April 17 (Reuters) - British wholesale natural gas prices dipped on Thursday morning as the system opened long, with mild springtime weather curbing demand and flows holding steady.

Gas for delivery on Friday slipped 0.60 pence to 50.75 pence per therm by 1000 GMT, while gas for delivery within the day was down 0.40 pence, also at 50.75 pence per therm.

Prices further along the curve also fell, with gas for winter 2014 delivery down 0.50 pence at 64.75 pence per therm.

"Lower demand forecasts for the rest of the month are pressuring prompt contracts," a gas trader said. "Strength in sterling is continuing reductions in prices further out."

According to National Grid (LSE: NG.L - news) data, the British gas system was roughly 8 million cubic metres (mcm) long on Thursday morning, with forecast flows seen at 202 mcm and demand at 194 mcm.

An unusually warm winter has pressured gas prices across Europe by curbing the use of gas in heating, and gas storage levels are significantly higher than in previous years.

Meteorologists say temperatures in Britain will remain warm into next week.

Offering some support for gas prices, however, Norwegian output will be roughly 24 mcm per day lower on April 19-21 due to a planned gas field shutdown.

Gas traders also remained nervous about the risk of a supply disruption from major gas exporter Russia, which supplies roughly a third of Europe's gas.

Moscow has threatened to cut off gas supplies to Ukraine, a major transit route for Russian gas to Europe, but so far the flows have been stable.

In Britain's power market, prices for baseload (24 hours) delivery on Friday rose 1.50 pounds ($2.52) to 42.20 pounds per megawatt-hour as wind power production was expected to decline. ($1 = 0.5955 British pound)

GAS UK natural gas prices National Grid instant flows National Grid gas demand North Sea gas and power maintenance

POWER UK: baseload prices , peakload prices , outages FRANCE: baseload prices , peakload prices , outages, report GERMANY: baseload prices , peakload prices , outages, report CENTRAL EUROPE: report SWITZERLAND: outages, reservoir levels NORDIC (SES: MR7.SI - news) : report EU CARBON PRICES SPEEDGUIDE:

(Reporting by Alexander Winning; Editing by Dale Hudson)