Boris Johnson has told the EU the Irish border backstop must be scrapped as it is “unviable” and “anti-democratic”.
In a letter to European Council President Donald Tusk, the PM also said the backstop risked undermining the Northern Irish peace process.
But Mr Tusk said those opposing the arrangement without “realistic alternatives” supported re-establishing a hard border.
This was the reality “even if they do not admit it,” he added.
“The backstop is an insurance to avoid a hard border on the island of Ireland unless and until an alternative is found,” Mr Tusk tweeted.