Hundreds of British Gas engineers will lose their jobs at noon Wednesday after refusing to sign pending new contracts for tougher employment conditions, dubbed the company of controversial layoffs and re-hires.

On April 1, the UK’s largest energy supplier handed layoffs to nearly 1,000 of its engineers who install and repair boilers and heating systems for the company’s nine million service customers.

Engineers were given a two-week grace period during which they could change their minds and sign contracts that would extend the working day along with shifts on weekends and holidays – or lose their jobs.

It is estimated that hundreds of engineers have signed contracts in the past two weeks, and by the end of Tuesday another 500 have refused to sign. The company expects the latest wave of 11-hour contract signings on Wednesday morning to leave 300 to 400 engineers jobless.

The end of the grace period is expected to end a bitter nine-month battle between British Gas executives and GMB union officials, who accused the supplier of “bullying” their employees.

The layoff and rehiring scheme is legal, but has sparked ire among employees and GMB. British Gas outlined its plans last summer as part of a formal union consultation process to streamline its contracts and increase productivity to help save businesses from financial ruin.

Under the new contracts, full-time engineers will be required to work an additional three hours per week, or 40 hours per week in total, and will not be paid a higher rate for work on weekends and holidays.

The terms were accepted by most unions and employees, but GMB has staged more than 40 days of strikes in recent months to protest the “mass layoffs” of its members.

Andy Prendergast, GMB’s acting national secretary, said that while many of its 8,000 engineers agreed to the new terms “under duress,” the company’s “horrific” treatment of its staff undermined employee morale.

British Gas said the company is changing the way it operates “to provide our customers with the service they want and to protect the future of our company and 20,000 jobs in the UK.”

British Gas owner Centrica has lost more than three-quarters of its market value in the past five years, and the supplier reported the lowest profit on record earlier this year.

“While change is difficult, we need to reverse our recession, which has resulted in the loss of more than three million customers, cut more than 15,000 jobs and halved profits over the past 10 years,” the spokesperson said.