South Northamptonshire MP Andrea Leadsom has resigned from her cabinet position tonight saying she does not 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".
In a resignation letter she said 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.
She went on to say "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."
Leadsom was a leading Brexiteer in the cabinet and one of a number of Brexit-supporting colleagues in the 'Pizza Club' - who were absent for the start of Prime Ministers Questions on Wednesday.
She resigned on a day that saw pressure mounting on the Prime Minister as she 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. 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."
Her decision has been criticised by the deputy leader of the Labour Party, Tom Watson, who said it was a 'slap in the face' to her Tory colleagues.