A cut in the maximum bet on fixed-odds betting machines (FOBTs) will not take effect until October 2019, according to plans being drawn up for next week’s budget, the Guardian understands, signalling a £900m windfall for bookmakers.

Details of the possible delay drew fury from campaigners and MPs, who have called for gambling companies to be given until April 2019 at the latest to prepare for a decision they learned of in May.

Former work and pensions secretary Iain Duncan Smith dismissed as “rubbish” claims by bookmakers that they needed more preparation time to limit job losses. The ex-Tory leader also accused the culture minister, Jeremy Wright, of being evasive when questioned about the issue during a select committee hearing on Wednesday, and hinted that any delay could spark a revolt among MPs.

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The government said it would cut the maximum FOBT stake from £100 to £2 after the chancellor, Philip Hammond, was persuaded by ministers at the Department for Digital, Culture, Media and Sport (DCMS) that the machines were a “social blight”. Hammond initially considered implementing the cut in April 2020 because of concerns about the £450m a year the Treasury derives from them in tax.

But he was understood to have chosen April 2019 instead following outcry about any delay, with any shortfall to be replaced by increased taxes on online gambling companies.

But well-placed Whitehall sources said the earlier date was recently removed from a budget plans, before being replaced with a compromise of October 2019. An extra six months would allow bookmakers to collect £900m from gamblers based on the current rate of winnings, meaning they will have racked up a projected £2.7bn since the stake cut was announced.

A Treasury spokesperson declined to comment, but one Whitehall source said Wright, who is understood to have met bookmakers after taking up the post in July, supported the later date.

In a select committee hearing on Wednesday, Wright told MPs that it was a “very good thing” that the maximum permitted FOBT stakes were being cut. But he said this should be weighed against measures bookmakers need to take “to reorder their business to make sure people aren’t made redundant if it can be avoided”.

He pointed to the industry’s argument that it needs up to 12 months to prepare, starting not from May’s decision, but from the day the government lays a statutory instrument after the budget. He did not say whether he had been convinced by the argument and insisted that he could not reveal information contained in the budget.

A DCMS spokesperson said: “We will make the legislative change required as soon as possible.”

“I thought he [Wright] was evasive,” said Duncan Smith. “It clearly sounds like we won’t see this until the autumn of next year.”

He said the countdown on bookmakers’ preparations should have started when the cut was first unveiled. “Once that decision was announced, there was no way back,” he said. “The idea they have to wait for the statutory instrument is complete rubbish.

“If they haven’t made the changes, it’s hard luck on them. They’re just playing for time, but there are people who are suffering.” He added that anti-FOBT MPs might have to “mount further action” if the deadline was pushed past April.

The former Tory leader has previously warned that he and colleagues would be “honour-bound” to seek amendments to the budget if the FOBT cut was delayed.

Labour MP Carolyn Harris, the chair of the all-party parliamentary group on FOBTs, said: “Any additional time is without justification and merely lining the bookmakers’ pockets.”

Matt Zarb-Cousin, a former gambling addict and spokesperson for campaign group Fairer Gambling, described any delay as inexcusable. “More people will get addicted and more lives will be destroyed as a result of any unnecessary delay,” he said.