The Foreign Secretary reportedly told the Czech daily Hospodarske noviny newspaper that, while the issue would be the subject of discussion, he did not believe the UK would be able to remain in the customs union.
“We probably will have to come out of the customs union, but that’s a question I am sure will be discussed,” he said, according to a translation of his comments by the Politico Europe website.
No 10 insisted that the Government’s position had not changed and that no decision had been taken on the future membership of the customs union.
“The Foreign Secretary reflected the Government’s position which is that a decision hasn’t been taken,” Prime Minister Theresa May’s official spokeswoman said. “He was very clear that the Government had not taken a decision.”
Officials insisted Mr Johnson had been clear that Britain’s future membership of the union was among a broad range of issues to be discussed and which had not yet been decided.
A Foreign Office spokesman said: “The Foreign Secretary was simply saying that the issue of customs union as well as many others will be a matter for discussion.”
Senior Labour MP Chuka Umunna - a member of Open Britain which campaigns for the closest links with the EU post-Brexit - said businesses would be “deeply concerned” by Mr Johnson’s comments and called on the Government to clarify its position.
“A decision on whether to leave the customs union is a huge choice that should be debated in Parliament, not decided off the cuff on a visit to Prague,” he said.
“Businesses will be deeply concerned. The customs union minimises bureaucracy at borders, which facilitates trade and business expansion. Outside, businesses could face costly delays, which in turn would hit growth and employment.
“The Government must now make clear what its policy is on the customs union and, if they want to leave it, produce rigorous analysis of how future trade deals would offset the very real costs to businesses of leaving it.”
Liberal Democrat leader Tim Farron said: “It is outrageous that Boris Johnson seems to think it’s OK for foreign newspaper readers to know more about our economic future than British businesses.
“Leaving the customs union will make the cost of doing business here soar and hurt our economy. The Government now needs to come clean about their plans for Brexit. The public shouldn’t have to sift through international newspapers to find out what our Government has in store for us.”