Robert Courts, 39, who won Witney in a by-election after David Cameron stood down, resigned from his role as a parliamentary private secretary at the Foreign and Commonwealth Office to allow him to vote against the Government tonight.
His exit follows the resignations of Boris Johnson, David Davis, Steve Baker and Greg Hands from ministerial jobs – and Maria Caulfield and Ben Bradley from party roles – over the Brexit row.
Mr Courts announced his departure on social media last night, expressing opposition to the Chequers Brexit plan.
He wrote: “I have taken very difficult decision to resign position as parliamentary private secretary to express discontent with Chequers in votes tomorrow.
“I had to think who I wanted to see in the mirror for the rest of my life. I cannot tell the people of West Oxon that I support the proposals in their current form.”
Mr Courts became a parliamentary private secretary, often seen as the first step on the frontbench ladder, in January.
At the time he said he was “delighted”.
He was re-elected with an increased majority last year and voted Leave in the EU referendum.