"Until now, no one has come up with anything wiser than that", Tusk told the Business Europe event.
Speaking at last month's "Brexit and the UK Energy Sector" event held in central London, Energy UK chief executive Lawrence Slade said that the I-SEM project was "absolutely critical" and described the situation on the island of Ireland as having "significant issues". Trade barriers would be especially damaging to Britain's fresh-food retailers, who rely heavily on the unencumbered movement of perishable goods throughout the EU.
She said both sides needed to accept that "neither side can have exactly what we want" but she was confident that an agreement can be found.
Julian David, the chief executive of British IT industry trade body techUK, said the group welcomed May's approach to allowing the ICO to "continue to play a role at an European Union level".
"If we look at our future prosperity and security, in the United Kingdom and in the other 27 countries, actually the right deal for us will be the right deal for them too".
"The Taoiseach is absolutely correct to say that the substantive negotiation is between the British government on one hand and the European Union member states collectively on the other and in our view it is important to maintain that dynamic", she said.
Her speech conceded that leaving the single market would have a major effect on the country, and proclaimed "life is going to be different".
"There won't be tripartite or three-way talks", he said.
Varadkar said this evening: "I didn't hear those comments today... but I can say I visited the US/Canada border back in August and I saw a hard border with physical infrastructure".
Regarding the Irish issue, she stressed that Britain cannot cope alone, and the European Union also has a responsibility to avoid a rigid border between Northern Ireland and the Republic of Ireland.
Mr Coveney said that if an agreement can not be struck, the backup plan of full British alignment with the EU's customs union and single market rules that Mrs May has "committed clearly" to would have to be enforced.
A no-deal Brexit would be catastrophic for the Republic. "It's the position of the parties in Northern Ireland, it's the position of the Irish government, and it was what we agreed in the December agreement of that joint report".
"What we want is not so much principles and aspirations and red lines", Mr Varadkar said.
Labour's shadow secretary of state for Scotland Lesley Laird said: "The Scottish Government needs to publish the areas of dispute it has with the UK Government over the EU Withdrawal Bill so the public can understand how we have got into this mess".
"We must build a new and lasting relationship while preparing for every scenario".