With Brexit plans in doubt, many British ministers have outlined the damage that they say a disorderly "no-deal" would do to the world's fifth-largest economy and its reputation as a politically stable haven for investment.
"We do not see compelling reasons to offer a different set of rules to EEA and non-EEA citizens unless the United Kingdom wishes to use migration in negotiations", the MAC said in a report commissioned by the government to assess the impact on the labour market of leaving the European Union and to make recommendations for a post-Brexit immigration system.
Writing in The Daily Telegraph, he said: "If the Brexit negotiations continue on this path they will end, I am afraid, in a spectacular political auto crash".
Britain would leave the European Union on March 29 2019 without a withdrawal deal covering expats' rights, the Irish border and the "divorce bill" and without agreement on future trade links.
Ireland's deputy prime minister will discuss efforts to avoid a hard border with the EU's chief Brexit negotiator on Tuesday.
When European Union leaders meet this week in the Austrian Alps, they will discuss how to clear both the last hurdles to a divorce deal and to a plan for future ties, he said. As part of the EU, Britain now enjoys the freedom of goods, services, money and people to move across borders with 27 other countries in the region.
"We do not see compelling reasons to offer a different set of rules to EEA (European Economic Area) and non-EEA citizens unless the United Kingdom wishes to use migration in negotiations", the report said.
Irish Taoiseach Leo Varadkar played down the prospect of Salzburg producing any shift in the EU's negotiating stance.
The Taoiseach told Ireland's Dail parliament: "I do not anticipate there will be any change to the EU's position or any change to our negotiating guidelines".
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In a high-profile Panorama interview on Monday, Mrs May framed the decision facing the country as a choice between her deal or no deal. May has repeatedly ruled out a second referendum.
But Theresa May told the BBC that if Parliament does not ratify her Chequers plan, "I think that the alternative to that will be having no deal". Labour's global trade spokesman Barry Gardiner said a second referendum would throw the Conservative government a lifeline.
"If we secure a People's Vote on the Government's final deal, Theresa May would be forced to resign".
Keir Starmer, Labour's Brexit spokesman, said any EU deal must meet Labour's key Brexit tests, which include delivering the "exact same benefits" as Britain now has inside the single market and customs union.
Speaking a week after predicting a Brexit deal in six to eight weeks, Mr Barnier said: "October is the key point in time - it is the moment of truth".
"The U.K and Japan are both champions of free trade", Hunt said, adding that they have been making "good progress" in creating a new bilateral economic relationship as agreed by their countries' leaders.
Mr Barnier has also been meeting European leaders in the run-up to the Salzburg summit.
Tusk also warns that a "no-deal scenario" remains "quite possible", but finishes his letter by saying: "If we all act responsibly, we can avoid a catastrophe".