* Spot gas prices up 5 pct on Langeled pipeline maintenance

* Spot power prices jump 10 pct on low wind, nuclear capacity

* Fear of Russian gas cut pushes up winter prices (Adds comment, detail, chart)

LONDON, Sept 1 (Reuters) - British wholesale natural gas and power prices jumped on Monday morning as reduced capacity tightened the system in the short term and the threat of a Russian gas supply cut to Europe over the Ukraine crisis pushed up winter prices.

Natural gas prices for delivery within the day and on Tuesday were both trading at 48 pence per therm at 0800 GMT on Monday, up around 5 percent each, and power prices for baseload (24 hours) delivery on Tuesday were up 10 percent to 44.85 pounds ($74.6) per megawatt-hour.

"Prices rose because tomorrow the Norwegian Langeled pipeline will stop sending gas to Britain for a day and because low winds will reduce renewable output at times of already low nuclear capacity," a utility trader said.

Norway's Langeled pipeline, which has the capacity to send 70 million cubic metres of gas a day to Britain, is scheduled to shut for one-day maintenance on Tuesday.

In power supplies, almost 3,000 megawatt (MW) of nuclear capacity is currently offline for maintenance and safety inspection while wind power capacity is expected to be almost zero for the few days.

Additionally, analysts said the increasing likelihood of a Russian gas supply disruption this winter over the Ukraine crisis was pushing winter gas prices up.

"We believe the Ukrainian situation will not be resolved without a transit interruption (and) prices would likely spike... as Europe would then have to compete for higher priced LNG to balance its demand," French bank Societe Generale (Paris: FR0000130809 - news) said.

The European Union threatened Russia with new trade sanctions if Moscow fails to start reversing its action in Ukraine.

Gas prices for December were up half a pence at 63.40 pence per therm.







(1 US dollar = 0.6012 British pound) (Reporting by Henning Gloystein; Editing by Michael Urquhart and Susan Thomas)