BART workers will go on strike Friday after failing to reach an agreement on a new contract during negotiations with the Bay Area transit system.

The strike, which will affect about 400,000 commuters, will take effect at midnight Friday.

A timely agreement Thursday afternoon could have averted the strike, but the unions rejected BART’s “final offer,” according to a press release on the BART website.

The offer included a 12 percent wage increase over four years, with a chance for employees to earn up to $1,000 if ridership grows, according to the release. The offer asks BART workers to contribute 4 percent of their earnings toward their pensions and 9.5 percent toward medical coverage.

BART gave the unions until Oct. 27 to consider the offer.

“This isn’t the end of it — it’s the beginning,” said BART General Manager Grace Crunican to news organizations outside the negotiating office in Downtown Oakland. “It’s not management that asked for the strike — it’s the union.”

Negotiations had continued over the past few days despite BART officials’ decision to propose their final offer Sunday. The employees are represented by two unions, Service Employees International Union Local 1021 and Amalgamated Transit Union Local 1555, which represent more than 2,000 BART employees in total.

According to a statement by Roxanne Sanchez, president of SEIU Local 1021, the unions and BART management have come to an agreement about economic issues such as wages.

ATU Local 1555 said in a Facebook post that although a broad consensus on many issues was reached, the deal was sunk by the transit system’s “late requests to eliminate worker protections.”

“We were close to a deal, but management kept moving the finish line,” said Antonette Bryant, president of ATU Local 1555, in a statement. “Now they have put a brick wall where the finish line should have been. They have left us with no recourse but to reevaluate our role in these unproductive negotiations.”