Anne Milton has resigned from the Government ahead of the new prime minister taking office due to “grave concerns” about a no-deal Brexit.
Former foreign secretary Boris Johnson, who is widely expected to win the Tory leadership contest and become the next prime minister, has repeatedly said he will leave the European Union on October 31 with or without a deal.
Quitting as education minister, Ms Milton said she wanted to return to the backbenches “to be free to do what I feel is right for the country” – a clear warning that she will oppose Mr Johnson if he pursues a no-deal strategy.
The Guildford MP defied the whip in order to abstain in a vote last week which saw MPs back moves aimed at making it harder for Mr Johnson to force through a no-deal Brexit by suspending Parliament.
In her resignation letter to Theresa May she said: “I have always believed that our departure from the European Union should be centred around future co-operation and I had sincerely hoped we would have been able to leave the EU in March with a deal in place.
“I regret very much that this was not possible.
“However I have grave concerns about leaving the EU without a deal, and so I feel it is time for me to return to the backbenches.
“It is important to me to be free to do what I feel is right for the country and my constituents.”
Her resignation follows that of Sir Alan Duncan from the Foreign Office on Monday.
More ministers – including Chancellor Philip Hammond and Justice Secretary David Gauke – are expected to quit before Mr Johnson takes office, if he wins the Tory leadership contest.