South Northamptonshire MP Andrea Leadsom has resigned from her role on Theresa May's cabinet saying she doesn't believe the Government's approach will deliver Brexit.
The Leader of the House of Commons said she couldn't support a deal that opened the door to a second referendum that would "undermine the union".
She also believes there has been "a complete breakdown of collective responsibility" in Mrs May's cabinet.
"I do not believe we will be a truly sovereign United Kingdom through the deal that is now proposed," Mrs Leadsom wrote in her resignation letter.
"I have always maintained that a second referendum would be dangerously divisive, and I do not support the Government willingly facilitating such a concession. It would also risk undermining our union which is something I passionately want to see strengthened.
"There has been such a breakdown of government processes that recent Brexit-related legislative proposals have not been properly scrutinised or approved by cabinet members.
"The tolerance to those in cabinet who have advocated policies contrary to the Government's position has led to a complete breakdown of collective responsibility."
Mrs Leadsom's resigned on a day that has seen pressure mount on the Prime Minister after Mrs May faced a backlash from Conservative MPs over her latest Brexit deal.
In her deal, the PM has offered MPs the chance to vote on whether to hold another referendum if they agree to back her Brexit deal.
"I cannot fulfil my duty as Leader of the House tomorrow to announce a bill with new elements that I fundamentally oppose," wrote Mrs Leadsom.
"No one has wanted you to succeed more than I have, but I do now urge you to make the right decisions in the interests of the country, this Government and our party.
"It is therefore with great regret and with a heavy heart that I resign from the Government."