Employees were given a deadline of midday today to accept the changes by signing new contracts. Those who don’t will lose their job.

The move comes as the energy giant faces a huge backlog of service and repair appointments following strikes by engineers. Engineers are on strike today – the 43^rd day of strike action in the past six months.

Union GMB said British Gas “doesn’t give a toss for either customers or staff”. It accused the company of “corporate bullying” and pressuring staff to accept a 15% cut in pay rates and other imposed changes in terms and conditions.

But British Gas owner Centrica denied that engineers faced a 15% pay reduction or a decrease in base pay. It said the new terms were mainly related to the way the company worked. It predicted that between 300 and 400 engineers would “make the choice” to leave.

British Gas announced in July last year that hundreds of staff who refused to sign new contracts would be sacked. GMB says the company bullied many staff to sign terms they had overwhelmingly rejected and still don’t accept.

The strikes have led to a backlog of millions of customers waiting for planned service visits and hundreds of thousands having to wait for emergency repairs.

GMB also claimed that British Gas had suspended sales of boiler insurance – something Centrica denies. All British Gas’ Home care products are still for sale on its website.

Justin Bowden, GMB regional secretary, said: “That British Gas doesn’t give a toss for either customers or staff is shown by the mass sackings of engineers it needs so badly for customer services that it has suspended the sale of boiler insurance cover. The ‘graveyards of vans’ returned by the sacked gas engineers shows this.

“These sacked gas engineers are badly needed by customers to clear the huge backlog of missed planned annual service visits and repairs. There is sadly nothing in law to stop corporate bullying by companies of their own staff to sign terms they don’t accept and sacking those who don’t submit to this bullying.”

GMB said the dispute will continue and become an official national lockout dispute. A lockout is a temporary, employer-initiated work stoppage in which employees are prohibited from returning to work when a contract expires and there’s no replacement contract.

Bowden said: “The British Gas leadership disaster reaches its low point today (April 14) with mass sackings of British Gas Engineers – in the only consistently profitable part of the company – by a management team too stupid to see the true value of a uniquely skilled and loyal workforce.

“With hundreds of thousands waiting in the backlog for service, customers have been treated as collateral and so, it seems, too will staff – as Mr O’Shea prepares to go down in history as the first major CEO to mass sack his highly skilled and qualified engineers.

“They are his most valuable asset. He will be universally condemned by politicians and public alike for doing so. The arrogant gamble has been lost. Any fool can start a war and, it seems, ruin a good business. History will not be kind to Mr O’Shea or the Centrica Board who failed to rein in him and his out-of-control leadership team.”

A statement on British Gas’ website titled important industrial action information said: “Unfortunately, due to strike action by some engineers on Wednesday the 14th of April, we’re only able to offer a very limited emergency service. During this time we’ll be prioritising our most vulnerable customers, but we’ll do our best to help everyone as quickly as we can. Thanks for understanding – we’re so sorry for any inconvenience this may cause you.”

A Centrica spokesperson said: “There is a job for everyone at the end of this process. We are changing the way we work to give our customers the service they want and protect the future of our company and 20,000 UK jobs. Today marks the end of the period for our employees to sign new contracts.

“These are highly competitive, and our changes are reasonable. Around 98% of the entire company has accepted and we hope the remaining, also sign and choose to stay. We have not cut base pay or changed our generous final salary pensions. Our gas service engineers remain some of the best paid in the sector, earning £40,000 a year minimum.

“While change is difficult, reversing our decline which has seen us lose over three million customers, cut over 15,000 jobs and seen profits halved over the last 10 years is necessary. The changes will also unlock our ability to grow jobs and hire 1000 green apprentices over the next two years.’’