European Council president Donald Tusk has responded to a call from Boris Johnson to scrap the backstop, as the row over how to deal with Northern Ireland and Brexit continues.

Writing on Twitter, he accused politicians who are against it and refusing to provide “realistic alternatives” as “in fact support[ing] reestablishing a border”.

“The backstop is an insurance to avoid a hard border on the island of Ireland unless and until an alternative is found,” Tusk wrote. “Those against the backstop and not proposing realistic alternatives in fact support reestablishing a border. Even if they do not admit it.”

Johnson previously set out his call for the backstop – the contingency plan to avoid a hard border with Ireland – to be scrapped from the divorce deal ahead of the October 31 Brexit deadline.

The prime minister had written to Tusk on Monday night outlining his opposition to what he called the “anti-democratic” Northern Ireland backstop.

In the letter, Johnson said while he wants the UK to leave the EU with a deal, he could not support any withdrawal agreement that “locks the UK, potentially indefinitely, into an international treaty which will bind us into a customs union and which applies large areas of single market legislation in Northern Ireland”.

European Commission spokeswoman Natasha Bertaud has also responded to Johnson’s proposal, pointing out that his letter “does not provide a legal operational solution to prevent the return of a hard border on the island of Ireland”.

“It does not set out what any alternative arrangements could be and in fact it recognises that there is no guarantee that such arrangements will be in place by the end of the transitional period,” she said. “Otherwise, as we have said on many occasions, we do stand ready to work constructively with the UK and within our mandate.”

Despite the backlash from Tusk, Downing Street is not backing down, issuing a fresh statement on Tuesday afternoon. 

In a note circulated to diplomats from the EU27, officials describe points made by Johnson in his letter to Tusk as “misleading” and “incorrect”.

The EU document takes issue with Johnson’s claims about the Good Friday Agreement and the Irish border. 

The briefing note said the Withdrawal Agreement “fully respects the Good Friday (Belfast) Agreement, the constitutional status of Northern Ireland and the territorial integrity of the United Kingdom”.

It was “incorrect to state that the people of Northern Ireland have no influence over the legislation that would apply to them” and the note highlights “specific governance arrangements” which could allow the UK and the people of Northern Ireland to express their views and to influence the shaping of the decisions.

The note continues: “The letter’s suggestion that two separate legal, political, economic, and monetary jurisdictions already exist on the island and can be managed with an open border is misleading.

“EU law provides the common framework needed to enable frictionless trade between Member States today. Without this common framework, checks and controls become necessary to protect consumers’ health, the integrity of the Single Market and Ireland’s place in it.”

Ireland’s premier Leo Varadkar had already rejected Johnson’s call to reopen the Withdrawal Agreement during a telephone conversation with the Prime Minister on Monday.

In response to the criticism, a Downing Street spokesperson said on Tuesday afternoon: “We are deeply invested in the peace, prosperity and security of Northern Ireland and always will be, and we have been clear that we will never place infrastructure, checks or controls at the border.

“But it is clear that unless the Withdrawal Agreement is reopened and the backstop abolished, there is no prospect of a deal.

“It has already been rejected three times by MPs and is simply unviable as a solution, as the PM’s letter makes clear.

“We are ready to negotiate, in good faith, an alternative to the backstop, with provisions to ensure that the Irish border issues are dealt with where they should always have been: in the negotiations on the future agreement between the UK and the EU.”

Johnson will meet German Chancellor Angela Merkel on Wednesday and France’s Emmanuel Macron on Thursday for his first face-to-face talks with Europe’s key powerbrokers.

The latest row with Brussels comes as a senior US politician warned that legislators would move to block a future US-UK trade deal if it puts the Good Friday Agreement at risk by introducing a hard border.

US Senate Democratic leader Chuck Schumer has written to US secretary of state Mike Pompeo warning that Congress could work on a cross-party basis to block a deal.

Schumer called for the Trump administration to stop “over-promising an unconditional and unrealistic” post-Brexit trade agreement with the UK.

Johnson spoke to Trump on the phone on Monday night to update him on Brexit ahead of their meeting at the G7 summit in France at the weekend.

Trump later said he and Johnson had shared a “great discussion” in which they talked about moving “rapidly on a US-UK free trade deal” after Britain’s departure from the bloc.