The sports minister Tracey Crouch has confirmed she is quitting her government role following a row over proposals to limit the stakes on gambling machines.
In her resignation letter, the MP issued a graphic warning of the consequences of a delay implementing the new curbs on betting machines.
She said the delays would mean £1.6 billion would be spent - a significant amount of which "in our most disadvantaged areas including my own constituency. 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 announced her decision in a statement saying: "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."
In a letter to the Prime Minister, the Conservative wrote there was never a good time to resign and realised it would come as an unwelcome distraction.
She claimed the changes were being delayed until October 2019 "due to commitments made by others to those with registered interests."
Political editor Paul Francis talks on KMTV about the resignation
But she told Theresa May: "Your personal support earlier this year for a reduction in the stake on fixed odds betting terminals was incredibly welcome and a reflection of your ambitions set out in your very first speech on the steps of Downing Street to support vulnerable people against the power of big business.
"I cannot begin to explain how many people got in touch to congratulate Government on its stance, including addicts, the families and also sadly those who have been left behind after loved ones took their own lives as a consequence of addiction."
She said between the time of the announcement and its actual implementation £1.6bn will be lost on gambling machines, a significant amount of which will be in deprived areas, including her own Chatham and Aylesford constituency.
She added: "In addition two people will tragically take their own lives every day due to gambling related problems and for that reason I believe this delay is unjustifiable."
Ms Crouch thanked the team of amazing civil servants she has worked with for the last three-and-a-half years.
She added: "I am very proud of what we have achieved in that time including the sports strategy, the gambling review, the Civil Society strategy and of course the most recent loneliness strategy, which I was humbled to lead on your behalf.
"I hope you understand my position and accept my resignation with the sadness it is tended.
The MP for Chatham and Aylesford was unhappy the Chancellor Philip Hammond has delayed the introduction of cutting stakes on fixed odds machines by six months.
The introduction of limits – cutting the maximum stake from £100 to £2 – will not come into force until next October.
Campaigners want the change brought in from April. This was rejected by Chancellor Philip Hammond in the Budget on Monday.
Earlier today there was growing speculation that the MP had already handed in her resignation but it had not been accepted.
Ms Crouch, who is returning from a trip to America, refused to rule out quitting over the issue.
There was an outpouring of messages on social media following the announcement from activists of all political colours.
Labour deputy leader Tom Watson described the move as a "courageous and principled decision".
He added: "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.
"The new Secretary of State has threatened all of this good work. He has prioritised corporate interests over victims, profits over public health and greed over good. He should be thoroughly ashamed."
Medway Labour leader Vince Maple also praised Ms Crouch for sticking to her principles, while the chief executive of UK Scouting, Matt Hyde, posted: "I am so sorry to hear Tracey Crouch has resigned. She was a terrific minister and totally committed to the value of civil society."
Meanwhile, chairman of the Digital, Culture, Media and Sport select committee and Folkestone and Hythe MP, Damian Collins (Con) said: "She will be a loss to the government, and leaves behind a legacy of reform, not least on gambling regulation and the sports strategy."
Mike Whiting, chairmen of Kent Conservatives, said: "Tracey is a conviction politician who acts on those convictions in everything she does.
"It is regretable she has been put in a situation where she feels it necessary to resign.
"She has been a superb and very well respected Minister and will continue to be an excellent representative for the people of her constituency in Chatham and Aylesford."
Ms Crouch was made Sports and Civil Society Minister shortly after the 2015 General Election.
She was appointed the world's first Minister for Loneliness in January.
She could not be reached for comment.
Political editor Paul Francis writes: "It is a measure of her standing among her peers that Tracey Crouch received supportive messages from not just her own party but other parties - both before and after her resignation.
Her decision to quit a job she described as the best in politics would have been hard to make but it was telling that in a tweet announcing her resignation, she spoke of politicians coming and going but that "principles stay with us forever."
Her involvement in the issue of the destructive impact of gambling machines was personal.
As she acknowledged in her letter to Theresa May, the government's backing for curbs on the amount people could bet was a key step towards stopping lives being destroyed by addiction. And it was something she had seen at first hand in deprived parts of her own constituency.
Some may ask why she has decided to quit not over a policy U-turn but over
a delay in implementing the measures. But it was clear that she felt the Treasury was contriving to delay unnecessarily the time scale.
Politically, her departure comes at an awkward time and has cast a shadow over a Budget which had been generally well received - at least by the Conservative party.
And many will wonder at the wisdom of Theresa May in not moving further to head-off the resignation of a minister who was well thought of across the political spectrum; personable and stuck to her guns.