The Forties oil pipeline - which carries 40% of North Sea oil and gas - has been shut down.
Operators Ineos said that a feed control valve closed at its Kinneil facility near Grangemouth at around 10.20am on Wednesday.
It is not yet clear when the pipeline will restart, and an investigation is under way.
Automated safety systems operated "as expected" to safely close the main pipeline system, which connects dozens of North Sea oil and gas installations with the mainland.
Ineos Forties Pipeline System (FPS) said it had now identified the issue and hopes to resolve it later on Wednesday and start up again overnight.
It is the second time the Forties pipeline has had to be shut down in recent months.
It was closed for around a fortnight in December following the discovery of a hairline crack.
A company spokesman said: "Ineos FPS confirms that a feed control valve closed at its Kinneil facility at around 10.20am today. Automated safety systems operated as expected to safely close the main pipeline system.
"We have now identified the issue that caused this and are hoping to resolve it this evening. We then plan to start-up again overnight."
The 235-mile Forties Pipeline System links 85 North Sea oil and gas fields to the UK mainland and the Ineos site in Grangemouth.
In 2016, the pipeline's average daily throughput was 445,000 barrels of oil and some 3,500 tonnes of raw gas a day.
The Ineos-owned pipeline pumps about 450,000 barrels of oil every day.
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