Britain risks its first recession in nearly a decade if it drops out of the European Union without a deal, according to Moody's.
And the government is proposing to cap any data charges at £45 a month.
According to the National Audit Office, there will be between 100,000 and seven million International Driving Permit (IDP) requests during the first year of Brexit, with applications being processed over the counter at the Post Office or by mail order from two private companies.
That means that the European regulation that bans roaming charges will not automatically be part of United Kingdom law, so British mobile operators might be able to reintroduce the charges.
The UK would "recognise our strict legal obligations" but that the amount paid would be "significantly, substantially lower" than the £39bn agreed with the EU.
The government is advising British drivers that they may need to get an International Driving Permit, which they would have to carry alongside their United Kingdom licence, to keep driving in the EU.
The CBI said the notices also showed the extent of disruption consumers can expect if "ideology wins over evidence".
Barnier tweeted that he had a "useful dialogue" with his British counterpart Dominic Raab on Friday morning about progress their teams had made toward a withdrawal agreement. "But, in a no-deal scenario, we can't control the EU's response to United Kingdom goods going the other way". Instead, officials hope the talks will give new momentum to the dialogue leading up to the regular European Union summit on October 18.
"We will be working closely with the Government and other European operators to try and protect the current arrangements so our customers can continue to enjoy free EU roaming once Britain officially leaves the EU".
The move could impact the 2.6 million private motorists and HGV drivers who head to European Union countries each year and would mean drivers may need to get one of two different types of IDP - depending on the country - or may even need both if travelling across borders.
CBI Director General Carolyn Fairbairn said the government's latest planning papers "make clear firms would be hit with a sledgehammer in the event of no-deal".
Although the United Kingdom is not a member, British citizens are now able to enter the Schengen area if they have a valid passport, and there is no requirement to have a certain amount of time left before expiry, the Home Office guidance says.
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