* J-Block gas field outage, cooler weather lift prices
* Concerns over gas transit from Russia to Ukraine
* UK offline nuclear capacity could reach 3 GW this week
LONDON, Aug 11 (Reuters) - British wholesale natural gas prices rose on Monday morning on the back of an unplanned outage at a North Sea gas field, lower temperature forecasts and concerns about the transit of gas from Russia via Ukraine.
Day-ahead British gas prices increased by 1.00 pence to 38.50 pence per therm by 0839 GMT. Gas for immediate delivery was up 0.30 pence at 38.00 pence per therm.
"The prompt has ticked up on the back of another outage at J-Block, lower temperature forecasts and bullish curve contracts," a gas trader said.
ConocoPhillips' North Sea J-Block natural gas field went into an unplanned outage on Sunday, the operator said.
The shutdown started at 3:20 p.m. local time (1420 GMT) and ConocoPhillips said an investigation was under way as to when the field would return to service.
The field's typical production rate is 10 million cubic metres per day.
Temperatures across Britain are also expected to fall below the seasonal average over the next two weeks, potentially increasing demand for gas.
Further along the curve, contracts firmed on concerns that possible transit sanctions by Ukraine against Russia could affect the supply of gas to Europe.
Gas for delivery in winter 2014-15 inched up 0.01 pence to 61.80 pence per therm, while gas for delivery in the fourth quarter was 1.00 pence higher at 59.00 pence per therm.
Russia supplies nearly a third of Europe's gas needs and around 5 percent of Britain's.
In Britain's power market, day-ahead baseload power (24 hours) was down 0.75 pound at 35.00 pounds per megawatt-hour.
EDF Energy said it would take three of its nuclear reactors in Britain offline for inspection after finding a defect on a reactor of a similar design.
That means unavailable nuclear capacity could reach nearly 3 gigawatts this week. (Reporting by Nina Chestney; Editing by Dale Hudson)