Mr Rees-Mogg said: “Before my Right Honourable Friend next goes Brussels, will she apply a new coat of paint to her red lines because I fear on Monday they were beginning to look a little bit pink.”
One of Theresa May’s Brexit red lines included the UK leaving the EU’s single market and customs union after Brexit.
But on Monday it was revealed in a leaked draft paper that Mrs May planned to have “continued regulatory alignment” between the Irish Republic and Northern Ireland.
The plan was shut down by Democratic Unionist Party (DUP) leader Arlene Foster, who said “any form of regulatory divergence” between the Irish Republic and Northern Ireland would be unacceptable.
Mrs May insisted she remains certain Britain will leave the single market and customs union after Brexit.
She said: “I can happily say to my Honourable Friend that the principles on which this Government is negotiating was set out in the Lancaster House speech. They were set out in the Florence speech and those principles remain.”