Amber Rudd has risked fuelling controversy over Britain's Brexit deal by suggesting that the question of UK membership of the customs union remains open.
The Home Secretary, who was a prominent supporter of Remain in the 2016 referendum, declined to say that the UK was certain to leave the customs union when pressed on the issue at a Westminster lunch.
Ms Rudd said the government was "still working on" its position, which would require further discussions with Cabinet colleagues.
Speculation that prime minister Theresa May could be forced into accepting a customs union arrangement has mounted since her defeat in the House of Lords last week, prompting reports that senior Brexiteers including Boris Johnson would not accept any dilution of her "red line" on withdrawal.
Thursday's Commons debate on a backbench motion to establish a customs union with the EU after Brexit is not binding on the government and was not attended by many pro-Brexit MPs, with Tories not whipped to take part in the vote.
But the prime minister faces a more serious challenge next month when the much-delayed Trade Bill returns to the Commons.
Asked whether it was more or less likely, in the current climate, that Britain would stay in the customs union, Ms Rudd said: "I'm not going to be drawn on that.
"We still have a few discussions to be had in a really positive, consensual and easy way among some of my Cabinet colleagues in order to arrive at a final position."
And challenged over which side she would back in Wednesday's Commons vote if she were a backbencher, she said: "I'm committed to the government's position, which to some extent we are still working on."
Labour's shadow Brexit secretary Sir Keir Starmer commented: "Amber Rudd appears to have let slip that discussions around the Cabinet table about negotiating a customs union with the EU have not in fact concluded."
Liberal Democrat MP Layla Moran said: "The government at the highest level has no idea what their plan for the customs union really is. They are utterly and completely shambolic. The government is an utter car crash."