Boris Johnson has clashed with his Irish counterpart over the backstop in their first phone call since the Tory MP became Prime Minister.

Mr Johnson told Taoiseach Leo Varadkar that he will approach Brexit negotiations in “a spirit of friendship” but reiterated that any fresh deal must see the backstop abolished, Downing Street said.

But Mr Varadkar told him that the emergency measure to prevent a hard border on the island was “necessary as a consequence” of UK decisions, the Irish government said.

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The call on Tuesday came after the pound fell to a two-year low as Mr Johnson’s new government hardened its tone over the likelihood of a no-deal.

Their first discussion, nearly a week since Mr Johnson became PM, also came after allegations the PM was snubbing the Taoiseach.

No 10’s account of the call said Mr Johnson warned that the UK will be leaving the European Union by the October 31 deadline “no matter what”.

But Mr Johnson reaffirmed his commitment to upholding the Good Friday Agreement and to “never put” physical checks or infrastructure on the border, according to a spokeswoman.

“The Prime Minister made clear that the government will approach any negotiations which take place with determination and energy, and in a spirit of friendship, and that his clear preference is to leave the EU with a deal, but it must be one that abolishes the backstop,” she added.

Dublin said Mr Varadkar reiterated the EU position that Theresa May’s Withdrawal Agreement would not be renegotiated.

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“On Brexit, the Taoiseach emphasised to the Prime Minister that the backstop was necessary as a consequence of decisions taken in the UK and by the UK Government,” a spokesman said.

“Alternative arrangements could replace the backstop in the future, as envisaged in the Withdrawal Agreement and the Political Declaration on the future relationship, but thus far satisfactory options have yet to be identified and demonstrated.”