LONDON (AP) _ More than 900 North Sea divers who take care of offshore oil and natural gas platforms went on strike indefinitely Wednesday after rejecting a 37 per cent pay rise over three years.

The divers are instead demanding a 50 per cent rise, claiming their earnings have slipped over the past two decades as oil and gas companies rake in ballooning profit.

The general secretary of the Rail, Maritime and Transport Workers union, Bob Crow, said the walkout would continue for as long as it took for employers to come back with better terms.

“This will have a huge impact on the oil and gas industry as the divers are needed to carry out essential repairs,‘‘ Crow said. “And there‘s no doubt that as the failures culminate it will inevitably hit production.‘‘

The employer signatories to Britain‘s Offshore Diving Industry Agreement said the workers had rejected one of the largest pay offers in modern times.

The U.K. Offshore Operators Association, which represents North Sea oil and gas producers, said Tuesday it hoped the strike would not reduce North Sea production, and “traditionally the winter period is a quiet time for diving on account of the weather.‘‘

Most planned North Sea field maintenance is done in the summer, but a prolonged strike could have an impact with workers unavailable for essential maintenance.

Major oil companies with operations in the North Sea said that oil and gas output had yet to be affected.

BP PLC said that it was monitoring the situation but that it had no plans yet to shut down oil or gas output.

The employers‘ offer to increase divers‘ salaries by 37 per cent over a three-year period was rejected by 68 per cent of the 756 union workers that voted in a poll that closed at midday Tuesday.

It follows the near-unanimous rejection of an earlier offer in September. About 79 per cent of the union‘s members voted in the most recent poll.

North Sea divers descend to depths of up to 40 metres in freezing water, carrying out installation and maintenance work on wells and pipelines.

Experienced divers can earn up to 46,000 pounds (US$87,800) a year but they have to pay for their own training, which can cost 20,000 pounds ($38,200). They retire at about 50.