Andrea Leadsom said, "I no longer believe that our approach will deliver on the referendum result," in her resignation letter.
Prominent Brexit supporter and leader of the House of Commons, Andrea Leadsom resigned from her post in Prime Minister Theresa May's government Wednesday, piling more pressure on the British leader after a new Brexit proposal angered members of her own party and fueled calls for her resignation.
So far May has resisted those calls, vowing to press on despite opposition from lawmakers and other ministers. She has softened her stance on a second referendum and customs arrangements in order to get her Brexit deal through parliament.
Leadsom said she could not announce the new Withdrawal Agreement Bill, which will implement Britain's departure, in parliament Thursday as she did not believe in it.
"I no longer believe that our approach will deliver on the referendum result," Leadsom, once a challenger to May to become prime minister, said in a resignation letter.
"It is therefore with great regret and with a heavy heart that I resign from the government."
A Downing Street spokesman praised Leadsom and expressed disappointment at her decision, but added: "The prime minister remains focused on delivering the Brexit people voted for."
May might still try to press on with her new Brexit plan, which includes a vote on whether to hold a second Brexit referendum — once her legislation passes the first stage — as well as closer trading arrangements with the EU.
But it has been met with a swift backlash, with several lawmakers who have supported her in previous Brexit votes saying they could not back the new plan, particularly over her U-turn regarding a possible second referendum.
"I have always maintained that a second referendum would be dangerously divisive, and I do not support the government willingly facilitating such a concession," Leadsom said.
"No one has wanted you to succeed more than I have," Leadsom wrote to May. "But I do now urge you to make the right decisions in the interests of the country, this government and our party."
Labour lawmaker Ian Lavery, chair of the opposition party, said the resignation underlined that "the prime minister's authority is shot and her time is up".
"For the sake of the country, Theresa May needs to go, and we need an immediate general election," he said.
Labour's call echoed those of many of May's own Conservatives, who say that a fourth attempt to get her deal approved by parliament should be shelved and she should leave office to offer a new leader a chance to reset the dial.
"There is one last chance to get it right and leave in an orderly fashion. But it is now time for Prime Minister Theresa May to go -- and without delay," said Conservative lawmaker Tom Tugendhat, chairman of parliament's Foreign Affairs Select Committee.
"She must announce her resignation after Thursday's European (Parliament) elections," he wrote in the Financial Times.
But while so much about Brexit is up in the air, what is clear is that May plans to stay for now.