Sports minister Tracey Crouch has resigned amid claims the government has "delayed" slashing the maximum stake on fixed-odds betting terminals (FOBTs).
The Chatham and Aylesford MP, a leading campaigner on the issue, stepped down following an announcement in the budget that the cut from £100 to £2 will not take place until October 2019.
Several backbenchers claimed they had been assured it was due to happen at the beginning of May.
Her departure is more bad news for Prime Minister Theresa May, who has had to contend with seven Cabinet ministers leaving their post since the 2017 election.
FOBTs, on which users can gamble away up to £100 every 20 seconds, have been dubbed the "crack cocaine" of betting machines by campaigners, who say they are dangerously addictive.
The betting industry has warned that thousands of people could lose their jobs as a result.
In her resignation letter, Ms Crouch claimed "implementation of these changes are now being delayed until October 2019 due to commitments made by others to those with registered interests".
"From the time of the announcement to reduce stakes and its implementation over £1.6bn will be lost on these machines, a significant amount of which will be in our most deprived areas including my own constituency," she said.
"In addition, two people will tragically take their lives every day due to gambling related problems and for that reason as much as any other I believe this delay is unjustifiable.
"The alignment of the stake reduction with an increase in remote gaming duty was a condition put on by the Treasury to provide fiscal neutrality but is not a technical necessity, so there is no reason why implementation cannot come in sooner than October."
It is with great sadness I have resigned from one of the best jobs in Government. Thank you so much for all the very kind messages of support I have received throughout the day. Politicians come and go but principles stay with us forever. pic.twitter.com/rD8bEbCQcK
— Tracey Crouch (@tracey_crouch) November 1, 2018
In response, Mrs May insisted there had been "no delay".
She said: "We listened to those who wanted it to come into effect sooner than April 2020 and have agreed that the changes should be in place within the year – by October 2019."
The PM defended the time-frame, saying "we must ensure that this change can be implemented in an orderly and effective manner to make sure it delivers the results we all want to see".
In her letter responding to Tracey Crouchs resignation, Theresa May insists there has been “no delay” in reducing the maximum stake on FOBTs: pic.twitter.com/whqfBWeF0T
— Alan McGuinness (@Alan_McGuinness) November 1, 2018
Iain Duncan Smith, the former welfare secretary, told Sky News he was "desperately sorry" to see Ms Crouch go, adding that the machines are "very addictive" and cause "massive problems".
Conservative MP Boris Johnson, who quit as foreign secretary in July over Brexit, wrote on Twitter: "Congratulations to @tracey_crouch who deserves huge credit not just for her campaign but for sticking up for her principles"
Shadow culture secretary Tom Watson told Sky News: "This is one of those of those occasions – rare these days – where a minister has genuinely resigned with integrity and on a point of principle.
"We're very sad to see Tracey Crouch go over this very serious issue."
Mr Watson claimed Culture Secretary Jeremy Wright had "sold his colleague down the river and should have stood up to big gambling".
Liberal Democrat MP Christine Jardine said: "These machines are a blight on our society and undeniably damaging to vulnerable people.
"Tracey Crouch knows this, so why doesn't the prime minister? Theresa May should come to the floor of the House and explain why this will not be implemented for another year."
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Mr Wright defended the decision not to slash their odds yet, saying betting companies' profits was not a consideration.
He repeatedly refused to confirm whether Ms Crouch had resigned during an urgent question in parliament earlier on Thursday, but did praise her for doing an "outstanding" job.