The walkout marks strike days 39 to 42 in the row over the company’s ‘fire and rehire’ threat.

Engineers have been told they must sign acceptance of a 15% pay cut – and other detrimental changes to terms and conditions – by 12pm today or face being fired.

According to union GMB, whose members will be walking out, workers who do not sign will not be paid lump sum payments of up to £4,500 and other so-called ‘protected terms’ offered by British Gas.

A GMB survey found that 87% of engineers said that the reduced times in which to conduct their work would make them less safe.

On Monday (29 March) British Gas will give formal notice of termination for staff who have not accepted the imposed changes. Then on Thursday (1 April) those who don’t sign up to the cuts will be sacked with pay in lieu of the up to 12 weeks’ notice that the company are required to give.

British Gas engineers previously downed tools in January, leading to a backlog of 150,000 callouts according to GMB. However, British Gas disputes this figure.

GMB members have already overwhelmingly and repeatedly rejected these imposed changes. The union has agreed to declare an official national lockout dispute between British Gas and GMB from 1 April unless the company backs down.

It also instructed the dispute leaders to consult on further strike dates and other appropriate action.

Justin Bowden, GMB national secretary, said: “Mr O’Shea – CEO of British Gas – has unilaterally created an April 1 cliff edge to sack his workers and is driving the company at high speed toward it.

“If Mr O’Shea goes ahead with this reckless action, GMB has agreed to declare an official national lockout dispute with British Gas from April 1. There will also be further strikes and other appropriate action in this deadlocked dispute.

“For the CEO to gamble the future of a profitable business on a strategic decision of mass sackings is illogical. It will leave everyone bewildered. It is all the more illogical in that from the recent talks at ACAS, Mr O’Shea apologised for undermining trust by accepting dodgy legal advice to go ahead with fire and rehire and promised never ever to go down this road again.

“There is still time to pull back. Mr O’Shea should do what’s right for the business, the customers and the workers and take the April 1 deadline off the table.”

A statement on British Gas’ website said: “Unfortunately, due to strike action by some engineers between Friday 26th March and Monday 29th March, 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.”