Cabinet Office Minister David Lidington has appeared to back Theresa May's stance on Britain leaving the customs union, following a suggestion by a senior colleague that the question of UK membership remained open.
Mr Lidington, effectively Mrs May's second-in-command, said Britain was willing to negotiate binding trade agreements with the EU but seemed adamant the UK would still come out of the customs union.
Home Secretary Amber Rudd on Thursday declined to say that the UK was certain to leave the customs union but Mr Lidington said the Government has been "very clear" that leaving the EU meant doing so.
Speaking on a visit to businesses on the Irish border he said: "We're very clear that when we leave the European Union we leave the single market and the customs union, but the Prime Minister has also said that she wants a customs agreement with the European Union.
"She said that she wants to get close alignment on industrial goods and that the United Kingdom is willing in the negotiations to consider binding agreements about future convergence over goods."
He said he had sought to reassure people in border areas that the Government was working "very energetically and with great determination" to resolve the border issue.
Mr Lidington said the Government was working towards "significant progress" on the complex border issue by June, and full agreement by October.
He met businesspeople in Newry and Dundalk who he said expressed "obvious, understandable concern" at any prospect of a hard border.
Speaking at pharmaceuticals company Norbrook in Newry after meeting the local chamber of commerce he said: "I gave a very strong assurance that the common travel area will continue, that's now been agreed by the UK and the EU together, so individuals doing their shopping, visiting doctors, have nothing to fear.
"And that we are working very energetically and with great determination on making sure that businesses do not face a hard border either.
"Prime Minster Theresa May has said repeatedly that ensuring no hard border on the island of Ireland is a fundamental strategic objective of our negotiating strategy and that remains the case."
His two-day visit to both sides of the border came in a week that began with an unannounced trip by Brexit Secretary David Davis.
Mr Davis faced criticism for not making public his visit and failing to inform local politicians he was travelling to the border.
The Brexit department later apologised, describing the lack of notification as "an administrative oversight".
Mr Lidington rejected a suggestion his colleague was wrong not to publicise the visit, saying: "There's no particular reason why he should seek a media opportunity out of it.
"His decision was to treat it as a fact-finding mission. That's something
that ministers do all the time."