Tusk says Johnson's plan will yield hard border

European Council chief Donald Tusk says that as long as Prime Minister Boris Johnson is not proposing "realistic alternatives" to the backstop agreement to avoid a hard border on the island of Ireland, he is actually backing "re-establishing a border."

A key part of the divorce proposals between the EU and Britain centres on keeping the island free of physical borders between EU-member Ireland and the Northern Ireland, which is part of the United Kingdom.

Both sides committed to a "backstop solution" to keep the border open in a deal with former prime minister Theresa May, but new prime minister Johnson vehemently opposes it.

Tusk tweeted that "those against the backstop and not proposing realistic alternatives in fact support reestablishing a border. Even if they do not admit it."

Boris Johnson has demanded that the European Union reopen Brexit negotiations, scrapping "anti-democratic" provisions for the Irish border that he says would threaten the peace process in Northern Ireland.

Johnson, who has made similar statements in the past, formally delivered his demands to the EU late Monday in a letter to Donald Tusk, president of the European Council.

Johnson is calling for an end to the so-called backstop, which 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.

Irish Prime Minister Leo Varadkar, after a one-hour call with Johnson on Monday, said the Brexit deal wouldn't be renegotiated.