Parents and grandparents who work in Amazon’s UK warehouses will in future be able to opt to work only in term time, amid a battle over pay and conditions with the GMB union, which is fighting the firm for recognition.

The US online retailer said the new contracts would guarantee those who care for school-age children, including guardians, time off during school holidays, including six weeks in the summer, plus two weeks at Easter and Christmas.

Amazon said it had trialled the contracts at three sites, after worker feedback, and these were being phased in across all of its fulfilment centres, where employees store, pick and pack products. They will be rolled out to sorting centres and delivery hubs across the UK this year.

The company said those on the new contract would be entitled to full-time benefits, and it hoped the move would encourage more people back to work.

It has also launched a flexible part-time contract, for a minimum of 80 hours a month, that allows employees to pick suitable shifts, whether part-day or full-day, during the daytime or night, and weekdays or weekends.

Amazon will hope the move can attract potential candidates looking for flexible employment, despite two recent rounds of job cuts that culled nearly 30,000 employees worldwide.

The offer of new contracts comes as Amazon is locked in a dispute with the GMB, after months of strikes at its Coventry warehouse.

GMB members at the huge Coventry centre, known as BHX4, have taken 16 days of strike action since January, in what was the first industrial action taken against Amazon in the UK.

They have been demanding a pay increase to £15 an hour, from £11, and have previously said they were frustrated by the company’s refusal to talk.

The GMB has said it represents a majority of workers at the Coventry site, and has made an application to a government body, the Central Arbitration Committee (CAC), for statutory recognition.

The union wrote to the CAC – the arm of the Department for Business and Trade that rules on union disputes – after Amazon rejected the union’s request for voluntary recognition.

The CAC has to make a decision over the size of the relevant bargaining unit, and whether more than half of the workers in it support the GMB’s request for recognition. If granted, this would be the first time a union in the UK has won the right to negotiate with the US tech company.

“We respect our workers’ rights to join or not to join a union,” said Neil Travis, Amazon’s regional operations director, in an interview with the BBC. “We are working with the CAC as part of the GMB’s formal application for recognition.”

Amazon has given workers a one-off £500 cost of living payment and two 50p pay increases in the past 12 months – the first after an informal walkout last summer and second this spring, taking pay to £11 an hour – although this has been labelled “an insult” by the GMB.

Workers have said the pay rises do not recognise that they worked throughout the pandemic, or offer them adequate support during a cost of living crisis that has pushed up their household bills.

Amanda Gearing, a senior organiser at the GMB, said: “It is no surprise that Amazon have discovered flexible working offers after Coventry workers have found their voice and their strength. It’s no coincidence that 16 days of strike action have come before this offer.”

The GMB has been steadily increasing its representation at Amazon UK over the past decade, and said it had signed up hundreds of new members at Coventry and other sites since the beginning of the dispute.

Amazon, which has a well-documented policy of non-cooperation with unions, has previously suggested the workforce is already able to communicate with senior managers.