The struggle to choke off a runaway well spewing 23,000 barrels of oil a day into the North Sea was stymied today by the discovery of a stunning week-old blunder.

A crucial piece of equipment to seal off the gusher -- the blowout preventer --down, officials disclosed. This weakened the "rams" or pistons used to close the hole, and they burst apart three times today under the impact of the oil jet as a disaster team attempted to stop the flow.

The blunder was discovered last Friday, but was not corrected then because the well was starting to blow. Since then, officials of the company responsible for the goof -- Moran Bros., a Wichita Falls, Tex., drilling contractor -- had concealed it from everyone except Phillips Petroleum Co., which owns the well.

Moran had been called in to install a blowout preventer after the well's operators had discovered an earlier blunder -- a tool that had been dropped into the well, preventing the operation of shutoff valves.

These are part of a number of tragi-comic errors that have marked the recent history of Bravo Rig's well 14. Unless the flow is cut off soon, the coasts of Norway, Denmark or Britain will be threatened with serious pollution. Norway's Environment Minister Gro Harlem Drundtland estimated tonight that the slick could reach some coastline in two to three weeks.

Robert Archambeault, Phillips' chief engineer in Norway, said of today's development: "It is a setback. Any setback is serious." Gordon Goering, the Phillips boss in Norway, said, "I'm disappointed but not dejected. There are other options open to us."

Norway's Labor Party government, which faces elections in September, is under heavy criticism for the affair, which newspapers here are headlining as a "great fiasco."

Today, challenged by a socialist member in Parliament, the authorities ordered all production in Phillips' Ekofisk field halted. The government insists that there is no danger from the other rigs, but it nonetheless felt compelled to make this political gesture.

A police spokesman here disclosed tonight that Stavanger authorities are investigating possible "criminal negligence" in connection with the disaster. The spokesman insisted, however, that this was a "routine" procedure.

Phillips, meanwhile, has decided to send Paul "Red" Adair, owner of the Houston oil well disaster company that is working here, to take a first-hand look at the gusher. Adair is due to arrive Friday, when another attempt will be made to stop the flow.

Moran company chief here, Frank J. Murphy of Denver, readily acknowledged that his crew had installed the blowout preventer upside down. He told reporters simply that the piece "looks the same right side up as it does upside down."

Moran said that when his men spotted the goof last Friday, the piece had already been partly bolted into place and tell-tale mud was oozing onto the deck of the rig. That indicated that the well was blowing, and 112 Phillips and Moran workers scrambled to safety.

After learning of the error, Archambeault said, Phillips tested an upsidedown preventer on shore and found that it leaked a little at a pressure of 4,000 pounds per square inch. The rams in a correctly installed preventer can normally withstand 5,000 pounds, and the blowout is estimated at 4,350 pounds.

If the preventer had been right side up, would the rams have sealed off the well? "There is every possibility," Archambeault answered.

Today a disaster team led by Asgar "Boots" Hansen of Adair's company tried three times to "kill" the well with the wrongway equipment.

The first try, near dawn, came tantalizingly close. The rams held for several seconds but then began to leak oil. Second and third efforts failed, too.