Theresa May was facing the threat of a fresh Commons Brexit rebellion despite meeting with senior ministers in an effort to forge a united front on EU withdrawal.
The Prime Minister and her senior cabinet held discussions at Chequers for eight hours to plot a way forward amid Brexit tensions within the Conservative party.
However she now faces a new challenge from pro-Europe backbenchers over keeping a customs union with the EU.
Leading Tory rebel Anna Soubry insisted on Thursday 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.
The move presents an increased danger to the PM because Labour's shadow foreign secretary Emily Thornberry said the party now backs a customs union that would look "pretty much like" the current one after withdrawal.
Ms Soubry said she had widespread support for her move, tweeting: "It would be in the national interest if the Government & official Opposition also backed it."
The Chequers "away day" saw the inner Cabinet committee discuss the impact of Brexit on the automotive sector, agri-foods, digital trade, as well as the overall future economic partnership the UK is seeking to reach with the EU.
Mrs 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 should work in what is being billed as a significant address on Monday, after some backbenchers called for more clarity from the leadership.
Ms Thornberry earlier made it clear Labour wanted close ties to Brussels, telling LBC: "Technically, because we are leaving the European Union we can't be in the customs union that we are in now.
"So, we leave and then we have to negotiate a new agreement. That we think is likely to be a customs union that will look pretty much like the current customs union."
The senior Labour figure also said the UK could join forces with Brussels to negotiate trade deals with third countries after Brexit, rather than make its own global arrangements.
"If we were, during these negotiations, to say to the European Union, if you want to negotiate with third parties, with third countries, we could be connected to that agreement, and it would be to the advantage of Europe that you have a great big economy like Britain as part of your negotiations."
The trade bill is not expected to be debated by MPs until after Easter.
