Tusk's apparently pre-planned remarks, which were tweeted from his account on the social media website as he spoke the words at the press conference, failed to impress Brexit-supporting politicians in Britain, where the message provoked some ire.
But many pro-Brexit British lawmakers fear the backstop will trap Britain in regulatory lockstep with the European Union, while May's political allies in Northern Ireland's Democratic Unionist Party say it imposes barriers between Northern Ireland and the rest of the United Kingdom.
"Ireland will not be the collateral damage of the Tory Brexit", she added.
Earlier in the day, Donald Tusk, the European Council president, reserved a "special place in hell" for Brexit campaigners in a furious outburst. (AAP) Irish Prime Minister Leo Varadkar and Donald Tusk.
May's spokesman stressed that she wasn't coming to Brussels to ask for more time and remained determined to deliver a Brexit deal before the March 29 deadline.
He said he no longer believed there was a way to stop Britain leaving due to the "pro-Brexit stance" of both the prime minister and the leader of the opposition.
The United Kingdom is on course to leave the European Union on March 29 without a deal unless Prime Minister Theresa May can convince the bloc to reopen the divorce agreement she reached in November and then sell it to skeptical British lawmakers.
The withdrawal agreement was rejected in the House of Commons last month, putting the United Kingdom on course for a no-deal Brexit. Labour Party foreign policy spokeswoman Emily Thornberry said such a postponement would allow "time to see whether the negotiations succeed or, if they do not, to pursue a different plan".
The eight week delay is a last-ditch attempt to solve the Brexit issue ahead of the European elections.
Brexit has snagged on the 310-mile (500-km) frontier because there is disagreement on how to monitor trade without physical checks on the border, which was marked by military checkpoints before the 1998 Good Friday peace agreement.
McDonald said the backstop "is the bottom line" for keeping the border open.
Juncker shot down that idea, and said: "We cannot accept the idea that has been circulated around that the withdrawal agreement could be reopened and as the backstop is part of the withdrawal agreement we cannot reopen the discussion on the backstop". "We will not gamble with peace; or put a sell-by date on reconciliation", Tusk said in a statement.
Saying Britain would leave as a "trusted friend" if it drops objections to giving Ireland a "backstop" guarantee on the border, Tusk's blunt language hinted at the hostility London may face if fails to find a compromise with its European neighbours.
He said: "I hope that tomorrow we will hear from Prime Minister May a realistic suggestion on how to end the impasse. following the latest votes in the House of Commons".