LONDON (Reuters) - A British union has filed a legal challenge against Amazon ( AMZN ), accusing the U.S. ecommerce giant of inappropriately influencing warehouse workers to vote against unionisation.

On Wednesday, warehouse staff in Coventry, central England, rejected a vote to become the first site outside the U.S. to force Amazon ( AMZN ) to negotiate labour terms. The GMB union narrowly lost the ballot, with 49.5% of the 2,600 workers who voted polling in favour, versus 50.5% against.

In a complaint to Britain's labour law regulator submitted on Wednesday, and seen by Reuters, the GMB union said Amazon ( AMZN ) had inappropriately quizzed workers on whether they were pro-unionisation, and placed posters around the site with QR codes which workers could use to cancel their union membership.

If the Central Arbitration Committee (CAC), an independent body that adjudicates on collective bargaining rights, accepts GMB's complaint, the body could order a fresh unionisation vote take place at the Coventry site.

An Amazon ( AMZN ) spokesperson said the company did not believe there was any legal merit in the GMB's complaint about the ballot process.

They said: "The recognition ballot was conducted by an independent third-party organisation appointed by the CAC. Their report contains no suggestion of any irregularities or unfairness. We are confident that a fair ballot process has been followed."

In a photograph filed with the complaint, a screen displays a QR code with the caption: "Why pay while you wait for a ballot? You can cancel your GMB membership anytime by scanning the QR code with your phone camera."

GMB said in its complaint that another 15 votes in favour would have resulted in victory, but its records indicated over 70 membership cancellation requests were received via the QR code.

The complaint is separate to an earlier legal action filed in April, an employment tribunal as part of which Amazon ( AMZN ) workers are seeking compensation, claiming they were inappropriately influenced to cancel their union memberships.

Rosa Curling, director at nonprofit Foxglove, which has been supporting the GMB union's organising effort at the warehouse, said: "This was a stitch-up, plain and simple."

She added: "We believe these tactics are unlawful and unfair, and call upon the CAC to uphold this complaint."

A spokesperson for the CAC declined to comment. (This story has been refiled to remove repetition of the phrase 'but its records indicated' in paragraph 8)