After Theresa May rejected EU proposals for Northern Ireland effectively to remain in the customs union, Mr Tusk warned that progress in Brexit talks could be put at risk if there was "backsliding" on principles agreed past year.
The recent suggestion by British prime minister Theresa May for a deal akin to that between the USA and Canada suggest something a little more intrusive than a few cameras reading vehicle licence plates.
Mike Cherry, national chairman of the Federation of Small Businesses, said discussions over future trade deals were "clearly important" but the priority for firms now was getting clarity about the immediate future with the European Union after Brexit day.
Mr Juncker said: "The 27 member states stand firm and united when it comes to Ireland".
EU Council President Donald Tusk warned last week that the Irish border issue must be solved before negotiations can move on to other issues. "Which means everything that we say and that we do on both sides will require the utmost clarity to pass the test".
Britain hopes to begin talks on the future trading relationship with Brussels next month, and May set out her proposals for a new wide-ranging free trade agreement in a speech on March 2. At its core was an aspiration that Britain would stick closely to European Union laws and regulations in certain sectors but be able to diverge in others.
Back in Brussels the day before, O'Leary of Ryanair said a stalemate in the Brexit negotiations points to serious disruptions for airlines.
In this regard, then, Ireland is the European Union, and there is little room for any independent negotiation to be had.
"Northern Ireland trade with Great Britain is worth almost 4 times more than Northern Ireland exports to the Republic".
Last year, she forced Ministers to give Parliament a say before the Brexit process was triggered.
After Donald Trump vowed to hike tariffs on imports of steel and aluminium, a move that is expected to hurt European businesses and the wider economy, Brussels threatened to retaliate with tariffs on U.S. products such as orange juice, motorbikes, jeans and peanut butter. "It's the United Kingdom that is closing doors", Barnier said.