JD Wetherspoon, the pub chain, has become the latest British company to offer staff on zero-hours contracts the opportunity to move to permanent hours.

Tim Martin, the founder and chairman of the company, told BuzzFeed on Sunday that a trial of offering staff guaranteed hours earlier in the year had proved so successful that it would be rolled out across the country.

Wetherspoon joins Sport Direct and McDonald’s in offering staff on casual contracts the opportunity to become permanent employees. The pub chain employs 24,000 people on zero-hour contracts, which is more than the controversial sports retailer Sports Direct.

Staff at Wethersoon will be offered guaranteed hours equivalent to about 70% of their typical working week. More than two-thirds of staff took up the offer during the trial.

“We’ve already offered guaranteed hour contracts to a percentage of our workforce and they’ll all be offered one in the next three months,” Martin said.

Meanwhile, Sport Direct employees who want to move to a permanent contract could be on zero-hours deals until the end of the year, according to an internal email.

The retailer has told workers that plans to move them to a contract with a guaranteed minimum of 12 hours a week “cannot be implemented for everyone overnight” due to the number of casual staff at the company.

More than 18,000 workers in Sports Direct shops are on casual contracts. Mike Ashley, the founder of the retail chain, has pledged to offer them a contract with guaranteed hours after a report into the company’s working practices by the law firm RPC.

The report was commissioned after the Guardian exposed how temporary staff at warehouses were in effect being paid less than the minimum wage. However, warehouse staff are not part of the initiative to move to contracts with a guaranteed 12 hours a week.

In a speech on Monday, the general secretary of the TUC will criticise companies who treat workers “like animals” and praise the role of trade unions in forcing Sports Direct to change its employment practices.

Frances O’Grady will say: “After months of Unite’s patient organising, winning public support and using trade union shareholder power, we got a result: an end to zero-hours contracts for retail staff, no more ‘six strikes and out’ and, at long last, the chance to get agency workers on to permanent contracts. A proper win for workers.

“Of course, it’s not over yet. Sports Direct may be in the spotlight now, but they are not the only ones. There are other big companies that bring shame on our country. So let me give fair warning to any greedy business that treats its workers like animals – we will shine a light on you.

“Run a big brand with a dirty little secret? A warehouse of people paid less than the minimum wage? A fleet of couriers who are slaves to an app? Let me put you on notice. There will be no hiding place. We will organise and we will win. Britain’s unions will not rest until every worker gets the fair treatment they deserve.”

In an internal memo to staff, seen by the Guardian, Leon Turnbull, head of people at Sports Direct, said casual workers would be given the opportunity to decide whether to become a permanent employee by the end of the year.

He said: “As you will appreciate, given the large number of casual staff engaged by the company, such a change cannot be implemented for everyone overnight. It is our intention that all members of the company’s casual workforce will be given the opportunity to elect to become an employee, or stay on their current terms, by the end of the year.

“We will provide you with further more detailed information on this proposal at the earliest opportunity and, in the meantime, thank you for your patience and understanding.”