Theresa May is facing the threat of a Commons rebellion on staying in the customs union after a marathon gathering of senior Cabinet members met to find a united front on EU withdrawal.
The eight-hour meeting of the Brexit "war cabinet" at Chequers was called to plot a way forward after Tory tensions went public, but the Prime Minister was threatened with a fresh challenge to her authority from pro-Europe Conservative backbenchers.
Former minister and leading Tory rebel Anna Soubry insisted she had cross-party support for a new amendment to the Government's trade bill which would mandate the UK to form a customs union with Brussels after Brexit.
Soubry said she had widespread support for her amendment, tweeting: "It would be in the national interest if the Government & official Opposition also backed it."
The move presents an increased danger to the Prime Minister because Labour's shadow foreign secretary Emily Thornberry said the opposition now backed a customs union that would look "pretty much like" the current one after withdrawal.
The Chequers "away day" saw the inner Cabinet committee discuss the impact of Brexit on the automotive sector, agri-foods and digital trade, as well as the overall future economic partnership the UK is seeking to reach with the EU.
May will set out the Brexit agenda in a major speech next week following a meeting of the full Cabinet.
Labour leader Jeremy Corbyn is to unveil a competing vision of how Brexit might work under a Labour government, in what is being billed as a significant address on Monday (February 26), after some backbenchers called for more clarity from the leadership.
Meanwhile, EU migrants who arrive in the UK during any transition period will be allowed to stay permanently, The Times has reported.
Such a move would represent a major climbdown by May after she insisted those arriving after March 2019 should not have the same rights as people who came before.