"The backstop is an insurance to avoid a hard border on the island of Ireland unless and until an alternative is found", Mr Tusk wrote.
British Prime Minister Boris Johnson fired the opening salvo in his bid to renegotiate Britain's divorce terms from the European Union, saying the backstop should be replaced with a pledge.
Meanwhile, UK Labour leader Jeremy Corbyn has called on Johnson to publish all documents on the impact of a no-deal Brexit, after a leak at the weekend suggested there could be significant disruption to supplies of food and medicine.
Johnson also warned that it risks "weakening the delicate balance" of the Good Friday agreement, as parties like the DUP are not happy with it.
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Writing to Mr Tusk, the PM confirmed he is committed to ensuring there is no return to a hard border on the island of Ireland.
However, Sinn Fein's leader in Northern Ireland, Michelle O'Neill, accused Johnson of "rank hypocrisy", adding that it was his "reckless pursuit of a no-deal Brexit that is threatening to undermine the peace process".
Johnson had written to Tusk outlining his opposition to what he called the "anti-democratic" Northern Ireland backstop.
But the European Union has long maintained that backstop must stay so as to ensure free movement and no hard border on the island of Ireland which is central to the Good Friday Agreement that has guaranteed peace on the island of Ireland.
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Planned Parenthood, along with other health organizations and about 20 states, continue to challenge the gag rule in court. A third part of the gag rule requiring clinics to physically separate those services will go into effect in 2020.
Johnson's letter "does not provide a legal operational solution to prevent the return of a hard border on the island of Ireland", she said.
He demands new alternative customs arrangements to be put in place in the two years of transition, then offers the possibility of "commitments" to prevent a hard border instead of the backstop.
The new resident of Number 10 spoke with his American counterpart in July, when Mr Trump said they were working on a "very substantial" post-Brexit trade agreement.
IT is probably premature - for the time being - to recall the Houses of Parliament this week over Brexit when little is likely to be achieved by such an exercise.
"I think that's very important because, of course, the Belfast Agreement has been used to justify the backstop but, as the prime minister points out, it doesn't have the support of ourselves - political unionism as we are so-called".
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