Sports minister Tracey Crouch has quit from government over a delay to proposed implementation of gambling reforms.
Ms Crouch is unhappy that a cap on fixed-odds betting terminal (FOBT) bets from £100 to £2 will not come in until October 2019.
She said in a statement: "It is with great sadness I have resigned from one of the best jobs in government," adding "politicians come and go but principles stay with us forever."
The move came after the government made it clear it was standing firm when Chief Secretary to the Treasury Liz Truss failed to give ground during the Budget debate in the Commons on Thursday.
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
Ms Truss told MPs that the government is "certainly happy to discuss" the matter, but gave no indication of a change of policy.
In her resignation letter to Theresa May, Ms Crouch wrote: "I know this will come as a unwelcome distraction but... 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."
Labour MP Tom Watson said she had taken a "courageous and principled decision" and added that her boss, Culture Secretary Jeremy Wright, "has prioritised corporate interests over victims, profits over public health and greed over good. He should be thoroughly ashamed".
He said Ms Crouch had "poured her heart and soul into a significant review of these destructive machines... and took the right decision for those suffering from problem gambling, their families and communities".
She poured her heart and soul into a significant review of these destructive machines, faced down a systematic lobbying attempt by the gambling industry and took the right decision for those suffering from problem gambling, their families and communities.— Tom Watson (@tom_watson) November 1, 2018
Earlier, the government said the reform had not been delayed and that its implementation was timed to coincide with other tax increases to ensure that there was no public finance shortfall. It also said the industry needed time to adjust.
Labour's Carolyn Harris earlier asked the front bench to confirm Sports Minister Tracey Crouch had resigned during the Budget vote.
Ms Harris, chair of the all party parliamentary group on FOBTs, said: "There are very sad reports the minister for sport has resigned as a direct result of the chancellor's Budget.
"Can a member of the Treasury bench confirm whether this is true, and if so, I would like to put on record my support for her work and to thank her for the work she has done.
"Can the chancellor confirm to the House this is the first time a member of the government has resigned during the votes on the Budget, as a direct result of the chancellor's policies?"
The prime minister's office said it could not confirm Ms Crouch's resignation.