He added that everyone should now focus on delivering that. Tusk's appearance alongside Varadkar was the latest signal that the bloc will not abandon Ireland, which fears both the economic and political impact of a hard border.
In the months following the Brexit vote, the European Commission chief, Jean-Claude Juncker, blamed Brexit on "40 years of lies" by British politicians.
"I'm sure that when he reflects on it he may well wish he hadn't done it", she told BBC Radio 4's World at One.
Britain will risk crashing out of 40 trade agreements spanning five continents in event of a no-deal Brexit, the Confederation of British Industry (CBI) warned Wednesday.
Mr Tusk faced calls from a Cabinet minister to apologise for his "disgraceful" and "spiteful" remarks, while Brexit-supporting MPs lashed out at the former Polish prime minister.
Mr Tusk said the European Union would insist on the Irish border backstop being included in any approved deal as a "top priority" to preserve peace.
The Northern Irish Democratic Unionist Party (DUP), which has propped up May's government since she lost her parliamentary majority in a 2017 snap election, said it wanted to get a deal agreed but the border backstop had to be replaced.
Still open to a solution?
The dam broke on Donald Tusk's pent-up feelings about the leaders of the Leave campaign.
Reaction, as they say, was swift and furious from the Brexiteers. But officials have low expectations ahead of the prime minister's visit to Brussels on Thursday. But the facts are unmistakable. At the moment, the pro-Brexit stance of the United Kingdom prime minister, and the Leader of the Opposition, rules out this question.