Echoing the EU's unwillingness to remove the Irish backstop, "I don't see any majority in the European parliament to remove the backstop from the Withdrawal Agreement", the EU's Brexit Coordinator Guy Verhofstadt tweeted out.
A spokeswoman said of the letter that, "it does not set out what any alternative arrangements could be" and "recognises that there is no guarantee such arrangements would be in place by the end of the transitional period".
The backstop was aimed at squaring the circle: it would keep the United Kingdom in a customs union with the EU until a better solution was found, and keep Northern Ireland aligned to the rules of the EU's single market. Even if they do not admit it'.
In which Simon Coveney endorses by saying that this has been the EU's position all along and it remains that way.
"By contrast, we welcome today's re-commitment from Donald Tusk to the integrity of the backstop".
The prime minister said that the backstop - the contingency plan to avoid a hard border with Ireland - should be removed from the divorce deal ahead of the October 31 Brexit deadline.
European diplomats expect little progress on Brexit until the British domestic landscape becomes clearer when parliament returns on September 3 - a point after which the opposition Labour Party has vowed to try to collapse Johnson's government.
It is not the first time Mr Trump has spoken of the potential for a trade agreement after Britain leaves the European Union, but came as a senior USA politician warned that politicians could block a future deal if the Good Friday Agreement is undermined.
A leaked report showed that the British government is preparing for widespread shortages of food, fuel and medicines in the event of a no-deal Brexit.
The EU document takes issue with Johnson's claims about the Good Friday Agreement and the Irish border.
The backstop would keep Britain closely aligned with the European customs union if the two sides can't agree on other ways to prevent the reintroduction of border checks on people and goods moving between Northern Ireland and the Republic of Ireland.
Johnson had written to Tusk outlining his opposition to what he called the "anti-democratic" Northern Ireland backstop.
In a note circulated to diplomats from the EU27, officials describe points made by Boris Johnson as "misleading" and "incorrect".
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.
Johnson's government insisted the Operation Yellowhammer information was out of date and that Brexit planning had accelerated since he became Prime Minister.