In his first address to the British parliament as prime minister, Johnson on Thursday said the Irish border backstop clause in Britain's divorce agreement must be struck out as a precondition for any pact with the EU to avoid the economic disruption of a departure with no deal.

But Varadkar said there would be no withdrawal agreement or subsequent trade agreement with Britain if it did not accept the backstop - an insurance policy that would provisionally keep Britain in a customs union with the EU, pending a better solution, to prevent the return of a hard border between the Irish Republic and Northern Ireland.

By calling for the abolition of the backstop, Johnson was effectively calling for a "no-deal" exit, Varadkar said.

"I hope the new UK prime minister hasn't chosen 'no deal' but that will be up to them," Varadkar told journalists during a break in the final Irish cabinet meeting before the summer recess, which was held in County Donegal, close to the Northern Irish border.

"The position of the European Union and the position of Ireland has not changed: The backstop is an integral part of the withdrawal agreement," he said.

"Without the backstop, there is no withdrawal agreement, there is no transition phase, there is no implementation phase and there will be no free trade agreement until all those matters are resolved."

Varadkar said he was looking forward to meeting Johnson soon and would listen to his position. But he said the decision on whether Britain would leave with or without a deal was ultimately one for London.

"'No deal' is a British threat. The only people who can cause 'no deal' is the United Kingdom government," he said.

Johnson has said he wants to avoid a "no-deal" Brexit and that, if it comes to pass, it will be due to a lack of willingness to compromise on the part of Ireland and the European Union.