The European Union is ready for a no-deal Brexit and Britain would suffer most under such a scenario, Brussels has claimed.
European Commission spokeswoman Natasha Bertaud said no-deal would never be the bloc’s preferred outcome.
Prime Minister Boris Johnson has promised to take Britain out of the EU on 31 October, with or without an agreement in place.
Speaking on Monday, Mr Johnson said Britain can “definitely” get ready for no-deal.
But the EU was decidedly more downbeat on the prospect of this happening.
“This will obviously cause significant disruption both for citizens and for businesses and this will have a serious negative economic impact,” Ms Bertaud said, referring to a no-deal Brexit.
“That would be proportionally much greater in the United Kingdom than it would be in the EU 27 states.”
She cited European Commission President Jean-Claude Juncker as having said that if it a no-deal Brexit happened “it is the British who will unfortunately be the biggest losers”.
The comments come after a British government document detailing the potential impacts of a no-deal divorce was leaked at the weekend.
According to the Operation Yellowhammer assessment published in the Sunday Times, Britain could be hit with a three-month “meltdown” at its ports, a hard Irish border and shortages of food and medicine.
Downing Street said on Monday that the dossier was “out of date” and the government was “making all necessary preparations ahead of 31 October”.
A “large-scale public information campaign” is expected to begin shortly, Number 10 said.
A spokeswoman said: “In relation to business we have been engaging widely and will continue to do so and that’s been significantly stepped up in recent weeks.
“We have published numerous documents on how businesses can prepare.
“You can expect to see further information being published in the coming weeks in relation to how the public… can further prepare and the changes that they are likely to see across a range of areas.”
Asked about the leaked dossier, Mr Johnson said: “If you look at the preparations the UK had made by March 29, we were very far advanced.
“Things then slipped back a bit, but we’re very confident that by 31 October we will be ready and that’s the crucial thing.
“I think that’s what people want. People are very confident themselves that they can do it.
“I’m not pretending that there won’t be bumps on the road, there will be. I said that on the steps of Downing Street.
“But if everybody puts their minds to it, I have absolutely no doubt that we can get ready.”
Mr Johnson will this week head to Berlin and Paris as he tries to secure a new Brexit deal.
During trips to Germany and France, the PM will tell Angela Merkel and Emmanuel Macron “there must be a new deal to replace the failed Brussels deal” – but if one cannot be struck, the UK is prepared to leave the EU without an agreement.
Number 10 said it expected there to be “very little discussion” of Brexit during the visit to Berlin on Wednesday and Paris on Thursday, with other topics to be the focus.
Discussions are expected to centre on the G7 summit in Biarritz, France, next weekend, with trade, foreign policy, security and the environment set to be on the table. Mr Johnson will also likely meet with US President Donald Trump for talks.