Brexit Secretary David Davis, who has been leading negotiations to leave the EU, has resigned from the government.
In his resignation letter, Mr Davis criticised the PM's Brexit plan - agreed by the cabinet on Friday - saying it would leave Parliament with "at best a weak negotiating position".
In her reply, Mrs May said she did not agree but thanked him for his work.
Junior minister Steve Baker quit shortly after Mr Davis - as Mrs May prepares to face MPs and peers later.
In his letter, Mr Davis told Mrs May that "the current trend of policy and tactics" was making it "look less and less likely" that the UK would leave the customs union and single market.
He said he was "unpersuaded" that the government's negotiating approach "will not just lead to further demands for concessions" from Brussels.
Mr Davis, who was appointed Brexit Secretary in 2016, said: "The general direction of policy will leave us in at best a weak negotiating position, and possibly an inescapable one."
In her reply, Mrs May said: "I do not agree with your characterisation of the policy we agreed at cabinet on Friday."
She said she was "sorry" he was leaving but would "like to thank you warmly for everything you have done... to shape our departure from the EU".
(Source: BBC)