LISBON/DHAKA, May 31 (Reuters) - Retailers and unions negotiating over a legally binding workers' safety accord in Bangladesh due to expire on Monday reached a tentative deal to extend it by three months, unions involved in discussions said, provided the 200 signatory retailers agreed on the extension.

The signatories - which include top apparel retailers like Zara-owner Inditex and H&M - have until June 10 to state whether they agree, a spokesperson for UNI Global Union, one of the unions involved, said.

Inditex and H&M did not immediately respond to requests for comment on whether they would agree to the extension.

Unions have said they will pull out of the organisation now running the accord if retailers do not commit to the legally binding portion of the agreement, obliging them to pay for fixes at supplier factories and banning them from working with these factories until they are deemed safe by accord inspectors.

"Without a legally binding agreement, no factory is safe," Babul Akter, president of the Bangladesh Garment and Industrial Workers Federation, said. "Bangladesh's garment industry will go back to square one."

The Accord on Fire and Building Safety in Bangladesh was signed in 2013 after the collapse of garment production complex Rana Plaza killed at least 1,100 people.

It created an independent body to run inspections at factories and obliged retailers to pay for any repairs, collecting $500,000 from each signatory per year to fund its activities.

Originally due to expire in May 2018, the accord was extended that year, culminating in the creation of a new organisation called the Ready-Made Garments Sustainability Council (RSC).

Governed by factories, unions and retailers, the RSC took over the accord's team and duties in June 2020 but did not immediately commit to extending the legally binding portion of the agreement beyond its expiration date of 31 May. (Reporting by Victoria Waldersee, Ruma Paul, Editing by Euan Rocha and Steve Orlofsky)