The 235-mile pipeline system links more than 80 North Sea field to the United Kingdom mainland and the Ineos site in Grangemouth.
The pipeline carries 450,000 barrels per day (bpd) of Forties crude oil, roughly equivalent to a quarter of the daily output of the entire North Sea basin, and handles a third of Britain's total offshore gas production.
Force majeure, which suspends a company's contractual obligations in the wake of situations that lie beyond its control, is extremely rare in the North Sea. Deirdre Michie, the chief executive at trade group Oil & Gas U.K., said the outage was a devastating blow to the regional energy industry.
"HSE is aware of the shut-down of the Ineos Forties Pipeline and a specialist pipeline inspector is visiting the site as we begin investigating the circumstances", an HSE spokeswoman said in an emailed statement.
U.S. West Texas Intermediate (WTI) crude futures were at $56.81 a barrel at 0123 GMT, up 21 cents, or 0.4 percent, from their last settlement. At morning market prices, Michie is underestimating the loss by about $2.5 million.
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Oil prices briefly touched their highest since mid-2015 at about $65 a barrel after the shutdown, which has cut off supply of the largest North Sea crude stream.
BP CEO Bob Dudley said of the sale that his company's strategy was now shifting to other parts of the North Sea.
"A number of fix options are being considered and progressed", Ineos said earlier on Wednesday in a statement.
Michie's group warned early a year ago that the North Sea energy sector was in for a long period of decline.
Nexen, owned by the China National Offshore Oil Corporation, said last month that Buzzard was producing between 160,000-170,000 barrels per day.