The European Commission published a document warning over the potential impact of no deal, from the ability to travel to the collection of tariffs and the need to safeguard EU standards and regulations.
Johnson, who was the figurehead of the Vote Leave campaign, warned the government not to "make the fatal mistake of underestimating the intelligence of the public", who he claimed would understand that the deal the prime minister is proposing is "Brexit in name only".
European Union officials and diplomats have welcomed last week's proposals as a welcome if overdue starting point for negotiations on an outline of post-Brexit relations that is to accompany a binding treaty on the immediate aspects of withdrawal.
He said Britain had become mired in a fog of self-doubt and that there had been a stealthy retreat over the last 18 months from what he called the "bright certainties" of May's earlier vision of Brexit. "We have time in these negotiations, we have changed tack once and we can change again", he said.
The Commission insisted that its advice to undertake contingency planning for the worst possible scenario was not "a sign of mistrust in the negotiations", which are supposed to deliver a withdrawal agreement and a political declaration on future relations by October.
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London's new Brexit minister said he was confident he could reach a deal, on his first trip to Brussels on Thursday as the European Union warned business to get ready for Britain crashing out of the bloc without agreed terms to cushion the economic disruption. I hope you share that conviction.
The EU executive also explicitly stated that "no progress" has been made in agreeing to a so-called "backstop" solution to the border issue on the island of Ireland, that would guarantee an invisible border at the EU's new external barrier between the Republic of Ireland and Northern Ireland.
May said talks had already started with Brussels based on the proposal, which was set down in a policy document last week, and referred to as "Chequers" for the prime minister's country residence where its terms were thrashed out.
The two men took part in a short discussion on the UK's white paper before dining together at the UK's embassy in Brussels.
His comments reflect numerous concerns that have been expressed by the pharma industry, which has invested heavily in the United Kingdom as the EMA has been headquartered in London for the past two decades.
He went onto say: "Everybody wants to have a withdrawal agreement, I'm still confident we can have one because the cost of not having a deal is disproportionally higher than having a withdrawal agreement".