DETROIT -- More than 49,000 members of the United Auto Workers went on strike Monday against General Motors, bringing more than 50 factories and parts warehouses to a standstill in the union's first walkout against the No. 1 U.S. automaker in over a decade.

Workers left factories and formed picket lines shortly after midnight in the dispute over a new four-year contract. The union's top negotiator said in a letter to the company that the strike could have been averted had the company made its latest offer sooner.

The letter dated Sunday suggests that the company and union are not as far apart as the rhetoric leading up to the strike had indicated. Negotiations resumed Monday in Detroit after breaking off during the weekend.

But union spokesman Brian Rothenberg said the two sides have come to terms on only 2% of the contract. "We've got 98% to go," he said Monday.

Wall Street did not like seeing the picketers. GM shares were down 4 per cent in afternoon trading to $37.29.

On the picket line Monday at GM's transmission plant in Toledo, Ohio, workers who said they have been with the company for more than 30 years were concerned for younger colleagues who are making less money under GM's two-tier wage scale and have fewer benefits.

Paul Kane, from South Lyon, Michigan, a 42-year GM employee, said much of what the union is fighting for will not affect him.

"It's not right when you're working next to someone, doing the same job and they're making a lot more money," he said. "They should be the making the same as me. They've got families to support."

Kane said GM workers gave up pay raises and made other concessions to keep GM afloat during its 2009 trip through bankruptcy protection.

"Now it's their turn to pay us back," he said. "That was the promise they gave."

UAW Vice-President Terry Dittes told GM that the company's latest offer might have made it possible to reach an agreement if it had not come just two hours before the union's contract with GM expired on Saturday night.

In the letter to Scott Sandefur, GM's vice-president of labour relations, Dittes wrote that the company waited too long to make the offer. GM issued a statement saying it wants to reach a deal that builds a strong future for workers and the business.

Dittes wrote that there are many important items left in the talks, including wage increases, pay for new hires, job security, profit sharing and treatment of temporary workers.