Britain's biggest oil pipeline shut for second time in two months

* Ineos says plans to restart pipeline overnight

* System previously shutdown in mid-December

* Oil and gas prices jumped after latest closure (Updates with planned restart, price reaction, context)

By Ron Bousso, Libby George and Julia Payne

LONDON, Feb 7 (Reuters) - Britain's biggest and most important oil and gas pipeline Forties was shut for a second time in two months on Wednesday, raising concerns about the reliability of infrastructure behind the world's top oil benchmark.

Pipeline operator Ineos said it aimed to restart the pipeline overnight.

The pipeline, which pumps about 450,000 barrels per day of oil, was previously shut in mid-December for about three weeks when a crack was discovered.

As a result, Ineos declared force majeure on exports, citing problems beyond the firm's control, and the outage pushed up global oil prices.

Ineos said in a note to customers on Wednesday that the latest shutdown was caused by an unexpected closure of the feed control valves on the pipeline supplying the Kinneil gas processing plant.

"When these valves close, then this requires FPS (Forties Pipeline System) to shut down the pipeline system and its customers in the North Sea," it said.

Forties is the biggest of the five North Sea crude grades underpinning dated Brent, a benchmark used for oil trading in Europe, the Middle East, Africa and Asia.

The system also carries a third of Britain's offshore natural gas output.

Oil prices jumped by about 50 cents a barrel following news about the pipeline closure, before later giving up those gains.

British wholesale gas prices rose on the shutdown, with gas for immediate delivery up 6 percent at 54.00 pence per therm.

The pipeline was originally built to bring oil ashore from the vast Forties field. Over time, more oil and gas fields, now in excess of 50, were linked to it.

Royal Dutch Shell (LSE: 0LN9.L - news) said production at its Shearwater platform in the North Sea had been shut down as a result of the outage. The Nelson field, which also connects to the pipeline, had already been closed for maintenance, a spokesman said.

Total (LSE: 524773.L - news) said it had to stop gas exports from its Elgin Franklin terminal due to the Forties pipeline outage. (Reporting by Julia Payne, Libby George, Ron Bousso, Ahmad Ghaddar, Susanna Twidale; Writing by Dmitry Zhdannikov; Editing by Dale Hudson and Edmund Blair)