Raab resigned from the cabinet claiming that May's plan threatened the integrity of the United Kingdom and he could not support an indefinite backstop arrangement where the EU held a veto over Britain's ability to exit.
"Above all, I cannot reconcile the terms of the proposed deal with the promises we made to the country in our manifesto at the last election, This is, at its heart, a matter of public trust. I cannot support the proposed deal, " Raab said.
After Raab, Britain's work and pensions minister Esther McVey quit Prime Minister Theresa May's cabinet.
"The deal you put before the Cabinet yesterday does not honour the result of the referendum", McVey wrote in a letter to May.
Britain's Secretary of State for Work and Pensions Esther McVey.
Junior Brexit minister Suella Braverman became the fourth British government minister to resign on Thursday in protest at a draft withdrawal agreement with Brussels.
Braverman said in a letter to Prime Minister Theresa May that "concessions" made to Brussels in the framework deal "do not respect the will of the people".
(With inputs from agencies)
Britain's work and pensions minister Esther McVey quit Prime Minister Theresa May's cabinet, the second senior minister to resign on Thursday over a Brexit deal agreed between London and Brussels.