Sports minister Tracey Crouch has resigned over "delays" to a crackdown on maximum stakes for fixed-odds betting machines.
Chancellor Philip Hammond said in Monday's Budget that the cut in stakes from £100 to £2 would come into force in October 2019.
Ms Crouch said pushing back the date was "unjustifiable" and it could cost the lives of problem gamblers.
She tweeted: "Politicians come and go but principles stay with us forever."
Prime Minister Theresa May said she was disappointed Ms Crouch had resigned but there had been "no delay in bringing forward this important measure".
The government has denied Labour claims that MPs had been led to believe the cut would come into force at the start of the next tax year, in April 2019. They suggested the cut had been intended to be introduced in April 2020.
But in her resignation letter, Ms Crouch said: "Unfortunately, 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.
"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."
She added: "It is a fact of government that ministers must adhere to collective responsibility and cannot disagree with policy, let alone when it is policy made against your wishes relating to your own portfolio."