France has issued a warning that trains and planes from the UK could be turned back following a no-deal Brexit unless there are proper preparations in place.
French Europe minister Nathalie Loiseau said action was needed to prevent “chaos” following a no-deal break.
Downing Street sought to play down her remarks, saying preparations were in hand to deal with “all possible scenarios” after Britain leaves the EU on March 29 2019.
The Prime Minister’s official spokesman said: "It is in everybody's interests for that not to happen.”
Asked whether, in the event of a no-deal Brexit, it was a "real possibility" Eurostar trains from London could be rejected and planes leaving Britain turned back, she said: "The consequences of a no-deal which you mentioned are correct.
"If we do nothing and if we reach no agreement, this is what would happen, among other examples."
She added: "This is the reason why we need to prepare for a no deal because we cannot wake up on March 30 and say to our fellow citizens and to our businesses 'we thought it would not happen so we are not ready'."
Ms Loiseau, who held talks with Brexit Secretary Dominic Raab, also made clear that under EU rules, Britain could not sign any new agreements with individual member states until it has actually left the bloc.
She said: "Until the moment you became a third country there are no bilateral agreements which can be signed regarding the issues that are covered by the European laws.
"What we are preparing - and probably what the British Government is preparing - is a list of unilateral measures taken by each and every member state in different sectors to make sure we don't turn March 30 into chaos.
"And then we would have to start discussing with the British Government on bilateral agreements on dozens of topics for each and every member state. It would take us years."
While Mr Raab said ministers had agreed a set of measures to manage the risks in the event of no-deal, he acknowledged there would be some disruption.
He told BBC news: "We need to be honest about this. In the event of a no-deal scenario, which is not what we want, we would face short-term risks and short-term disruption.”
The warning came as the Government released its latest tranche of 28 technical papers on the no-deal preparations.
They include advice to motorists they may need to obtain International Driving Permits to continue driving on the other side of the Channel if the EU refuses to recognise UK licences.
British drivers who fail to obtain the correct documents may be turned away at borders or face enforcement action, the papers warn.