UPDATE 3-Norway gas output from Troll field shut due to leak

* Troll A, Kvitebjoern, Visund gas output shut after leak

* Oil output from Troll B and Troll C platforms reduced

* Oil output at Visund not affected

* UK spot gas prices jump nearly 12 percent in early trade

(Adds comments from Statoil, UK flow data)

By Gwladys Fouche and Kwok W. Wan

OSLO/LONDON, June 29 (Reuters) - Natural gas production at Norway's biggest field, Troll, and two adjacent North Sea fields are down after a gas leak early on Tuesday, Norwegian oil and gas producer Statoil (STL.OL) said.

Statoil said it has shut production at the Troll A platform, the Kvitbebjoern field as well as gas output at Visund.

Output from the Troll B and Troll C platforms, which produce oil, was also reduced, Statoil said without specifying volumes. Oil output from the Visund field was not affected, it added.

"We are still investigating the cause of the incident and it is too early to say when production will resume," spokesman Gisle Johanson told Reuters. "There will be no more information about Troll A today."

Statoil did not have to evacuate Troll A and said that the leak in the riser area of the platform was brought under control.

Johanson declined to comment on output volumes, saying: "We don't inform about the daily output from our platforms."

Norway's Petroleum Safety Authority, the safety watchdog for the oil industry, said it received a message about the incident at 7.45 a.m. (0545 GMT), about 20 minutes after the incident.

"There was an incident on Troll A ... It was a gas detector," Petroleum Safety Authority spokesman Ole-Johan Faret told Reuters.

The incident at Troll A means that gas was not feeding into Norway's Kollsnes plant, which processes close to 40 percent of total Norwegian gas exports, plant operator Gassco said, sending UK spot gas prices up nearly 12 percent to a high of 46 pence per therm on Tuesday. [NG/GB]

"We have full availability at Kollsnes, but the feed is missing from the fields," a Gassco spokeswoman said. "When we don't have any feed, we don't have anything to process."

She declined to comment on the output of the Kollsnes plant or how much feed was reduced by. Kollsnes has a total export capacity of 143 million cubic metres (mcm) of gas per day.

Gas flows into Britain via Norway's Langeled gas pipeline fell from around 60 mcm/day to around 27 mcm/day on Tuesday morning, according National Grid data. [ID:nWLA7250] Kollsnes also pumps processed gas to continental Europe.

The Norwegian Petroleum Directorate (NPD) said gas production from Troll, which uses the A platform, amounted to 11.66 billion cubic metres over the first four months of 2010, dropping to 1.60 billion in April from more than 3 billion per month in January-March.

The Troll B and C platforms are focused on oil production, which amounts to some 122,000 barrels per day, according to the NPD. Statoil did not say by how much production was cut.

For a FACTBOX on the affected fields, click on [ID:nLDE65S0KZ] (Additional reporting by Richard Solem and Wojciech Moskwa in Oslo, Daniel Fineren in London, editing by Anthony Barker)