Bargainers for the United Automobile Workers union and General Motors worked through the night in an effort to reach a settlement in contentious contract talks, facing an 11 a.m. Eastern time strike deadline on Monday. G.M.'s 73,000 workers began returning to factories around the United States this morning as scheduled, but were prepared to walk off the job when told to leave by their union. Picket signs, which had been printed 10 days ago when the U.A.W. and G.M. faced an original deadline of midnight Sept. 14, were brought out of storage at local union halls around the country. In many locations, assignments and schedules for picket duty have already been announced, only to be put on hold as negotiations went on.

One local in Flint, Mich., told workers on its Web site, "U.A.W. leaders set 11:00 a.m. as strike deadline. If a strike should occur, members are asked to refer to the letter handed out earlier," the notice, from Local 599, read.

Negotiations continued after the unexpected move by the union late Sunday night. Until then, the U.A.W. had extended its contract on an hour-by-hour basis, and the union's president, Ron Gettelfinger, told workers in a memo last week that the U.A.W. hoped to avoid a strike against G.M.