Since becoming prime minister last week, Johnson has spoken to several EU leaders but he has yet to say when he will make his first visits abroad.

"The PM has been setting out to European leaders the position ... that the Withdrawal Agreement with the backstop has not been able to pass parliament on the three occasions it was put in front of parliament. Therefore it needs to change," the spokeswoman said on Monday.

"The prime minister would be happy to sit down when that position changes. But he is making it clear to everybody he speaks to that that needs to happen," she told reporters.