Unions united in anger at the weekend after San Francisco rail bosses reneged on a signed agreement.

San Francisco Bay Area Rapid Transit (Bart) directors have ordered managers to reopen talks with ATU Local 1555 and SEIU Local 1021.

The board claimed a provision giving its 2,300 workers up to six weeks paid medical care leave each year would be too expensive.

"We are not comfortable with the potential liability that could result from the adoption of this contract provision," the board said on Friday night.

It was "never the intention to include the Family Medical Leave Act proposal in the contract," it added, claiming that the provision was "erroneously" included in the contract by an unnamed temporary employee.

By Bart's accounting only 7.4 per cent of its ATU and SEIU workers have taken leave for family care, at an average of 4.3 weeks, costing about $1.4 million (£870,00) a year.

But on Friday it claimed that a third of the union employees might take six weeks paid leave.

SEIU Local 1021 executive director Peter Castelli responded sharply that "there was no confusion or glitch in the agreement.

"Bart's high-priced chief negotiator Thomas Hock, assistant general manager Paul Oversier and labour relations manager Rudy Medina signed an agreement that would allow workers time off to care for their family members with a serious illness."

ATU Local 1555 president Antonette Bryant also criticised the backtracking.

"Today the Bart Board of Directors chose to walk away from a deal it negotiated over a six-month span.

"We are angered, shocked and disappointed."

"An agreement is an agreement. Bart just has buyers' remorse," Ms Bryant said.

"They could and should have identified any problems before 2,300 union members voted on the contract."

Mr Castelli added: "It's a further demonstration of incompetence and no real desire to get an agreement

"I've been bargaining union contracts for over 25 years and I have never seen anything like this happen, ever."

The contact ended a dispute that had caused two strikes.