Boris Johnson and his Irish counterpart Leo Varadkar have had their first exchange of views on Brexit in their first phone call since the former Mayor of London became Prime Minister.
Mr Varadkar reiterated that the backstop - the mechanism to avoid an Irish hard border - was needed because of decisions made by the UK.
The taoiseach also invited Mr Johnson to Dublin to discuss Brexit.
The PM again said the backstop must be removed from any deal with the EU.
He insisted the UK will be leaving the EU by the 31 October deadline "no matter what".
But Mr Varadkar maintained there could be no reopening of the withdrawal agreement.
A UK government spokesperson said that during the call both leaders had reiterated their commitment to work together in the spirit of the "warm and deep relationship" the two countries share.
A spokesperson for Mr Varadkar said he had explained to the new prime minister that the EU was "united in its view" on the withdrawal agreement.
Mr Johnson repeated his commitment that the British government would "never put physical checks or infrastructure on the border".
He said any further Brexit negotiations would be approached by his government in a spirit of friendship - but that any deal must be one that "abolishes the backstop".
What is the backstop?
The backstop is a key piece of the Brexit deal dictating what will happen to the border between Northern Ireland and the Republic of Ireland.
It is a last resort that guarantees a frictionless border if no better solution is devised in time - by maintaining close ties between the UK and the EU until such a solution is found.
The two men also discussed attempts to restore the Northern Ireland Assembly, which collapsed in January 2017.
Mr Johnson said the current talks have his "unequivocal support" and that he looked forward to visiting Northern Ireland shortly to talk to the leaders of the five main parties about restoring devolution.
The taoiseach restated the need for both governments to be "fully committed" to the Good Friday Agreement and restoring the institutions.
He invited Boris Johnson to Dublin to "share their respective analyses" on Brexit, and to continue discussions about Northern Ireland, the Good Friday Agreement and the Common Travel Area.