Workers at Amazon’s Coventry warehouse have voted to take a further six months of strike action, despite failing to win formal union recognition from the tech company.

Results of the ballot came as the GMB union’s 800 members at the site, known as BHX4, were on strike for the 19th day.

The union said 99% of its members had backed extending the strike campaign, on a turnout of 54%. Trade unions must renew their legal mandate for strike action every six months.

The GMB organiser Amanda Gearing said the vote showed staff were “in it for the long haul”, adding: “These workers are angry, they know their rights and they will not go away.”

Strike votes at two other Amazon warehouses, in Mansfield and Rugeley, failed to meet the required threshold, dashing the union’s hopes of concerted action over the summer. In Rugeley, they fell short by four votes, however, and members are considering a re-ballot.

The fresh vote for strikes in Coventry came after the GMB reluctantly withdrew an application for trade union recognition at BHX4 last week.

With membership at about 800 after a recruitment drive, the union believed it had comfortably signed up more than half of the staff at the site.

But the Central Arbitration Committee (CAC), which can grant statutory recognition, accepted evidence from Amazon that there were up to 2,700 workers employed there.

Amazon has rejected the GMB’s claim that it deliberately hired more than 1,000 staff in recent weeks to ensure the union would not be able to show that it had majority support.

The GMB claims its strike action – which began in January when it had only a few dozen members in Coventry – is having a growing impact on Amazon’s operations.

Since the industrial action began, Amazon has increased pay for UK-based staff to £11 an hour, and enhanced employment rights for parents; but GMB members in Coventry are calling for £15 an hour and union representation at company discussions.

The Labour leader, Keir Starmer, told the GMB’s annual congress in Brighton last week that he supported the union’s move for recognition at Amazon, calling it a “fantastic campaign”.

A spokesperson for Amazon said: “We respect our employees’ rights to join, or not to join, a union. We offer competitive pay, comprehensive benefits, opportunities for career growth, all while working in a safe, modern, work environment.”