* First and third shifts vote in favor of strike
* Second shift due to vote Wednesday afternoon
* Workers seeking 12 pct pay raise, shorter work week
SAO PAULO, April 24 (Reuters) - Workers at General Motors' Gravataí Industrial Complex in southern Brazil voted to strike on Wednesday over demands for higher pay and a shorter work week.
Workers from the plant's first and third shifts have voted in favor of the strike, and second-shift workers are expected to approve the measure at a Wednesday afternoon assembly, according to Valcir Ascari, head of the Gravataí Metalworkers' Union.
The strike, which has no set end-date, would halt production of the Chevrolet Celta, Prisma and Onix models at the Gravataí facility, which produces 1,200 cars per day, he added.
The plant's 4,500 workers are seeking a 12 percent pay increase and a reduction in working hours to 40 per week from 42.
GM had offered the workers an 8.29 percent raise at a Tuesday meeting and said it would gradually cut the work week to 41 hours by next January, Ascari said.
A GM press representative could not confirm whether production had stopped at the plant on Wednesday morning, and was unclear on whether the strike was a certainty.
The potential strike highlights GM's labor troubles in Brazil, where a tight job market has forced companies to agree to steep wage increases.
Since last year, GM has faced pressure from President Dilma Rousseff to retain workers in exchange for tax breaks that helped lift sales in the world's fourth-largest car market to a record high in 2012.