Irish premier Leo Varadkar has struck down one of the cornerstones of Boris Johnson’s plan to leave the EU with a deal by 31 October ahead of a meeting between the two leaders next month.

Last week the Irish Taoiseach invited Mr Johnson to visit him in Dublin to discuss Brexit and other issues between the two countries, but last night Mr Varadkar’s office insisted once again that there would be no movement on the Irish backstop – stating the point “has repeatedly been made clear”.

A spokesman added: “The withdrawal agreement and the backstop are not up for negotiation.”

He went on to reaffirm that the prime minister could seek to tweak the political declaration – a second document which outlines the future relationship between London and Brussels - but such discussions "would occur between the UK and the EU" at large.

The comments echo the sentiments of EU officials, who have argued the withdrawal agreement was set in stone since its establishment after two years of negotiations.

Biggest lies told by Boris Johnson Show all 5 1 /5 Biggest lies told by Boris Johnson Biggest lies told by Boris Johnson Made-up quote for The Times Johnson was sacked from The Times newspaper in the late 1980s after he fabricated a quote from his godfather, the historian Colin Lucas, for a front-page article about the discovery of Edward II’s Rose Palace. “The trouble was that somewhere in my copy I managed to attribute to Colin the view that Edward II and Piers Gaveston would have been cavorting together in the Rose Palace,” he claimed. Alas, Gaveston was executed 13 years before the palace was built. “It was very nasty,” Mr Johnson added, before attempting to downplay it as nothing more than a schoolboy blunder. PA Biggest lies told by Boris Johnson Sacked from cabinet over cheating lie Michael Howard gave Boris Johnson two new jobs after becoming leader of the Conservatives in 2003 – party vice-chairman and shadow arts minister. He was sacked from both positions in November 2004 after assuring Mr Howard that tabloid reports of his affair with Spectator columnist Petronella Wyatt were false and an “inverted pyramid of piffle”. When the story was found to be true, he refused to resign. PA Biggest lies told by Boris Johnson Broken promise to boss In 1999 Johnson was offered editorship of The Spectator by owner Conrad Black on the condition that he would not stand as an MP while in the post. In 2001 he stood - and was elected - MP for Henley, though Black did allow him to continue as editor despite calling "ineffably duplicitous" PA Biggest lies told by Boris Johnson Misrepresenting the people of Liverpool As editor of The Spectator, he was forced to apologise for an article in the magazine which blamed drunken Liverpool fans for the 1989 Hillsborough disaster and suggested that the people of the city were wallowing in their victim status. “Anyone, journalist or politician, should say sorry to the people of Liverpool – as I do – for misrepresenting what happened at Hillsborough,” he said. PA Biggest lies told by Boris Johnson ‘I didn’t say anything about Turkey’ Johnson claimed in January, that he did not mention Turkey during the EU referendum campaign. In fact, he co-signed a letter stating that “the only way to avoid having common borders with Turkey is to vote Leave and take back control”. The Vote Leave campaign also produced a poster reading: “Turkey (population 76 million) is joining the EU”

Asked in June how Boris Johnson’s premiership might impact the future arrangement between London and Brussels, European Council president Donald Tusk said: “Maybe the process of Brexit will be even more exciting than before because of some personnel decisions in London, but nothing has changed when it comes to our position”.

The backstop is woven into the withdrawal agreement brokered under Theresa May, and would see the UK enter into a customs agreement in the EU, while Northern Ireland would continue to operate under the rules of the single market.

EU representatives have argued it is the best way to avoid a hard border on the island of Ireland, ensuring the free movement of people and goods, and keeping in line with the Good Friday agreement which signalled the end of years of sectarian conflict in the area in 1998.

It proved to be a thorn in the side of Ms May, who lost four successive commons votes on the agreement with many of her own party members citing the backstop, which will last for an indefinite period of time, as a key reason for their dissent.

The issue, which played a key role in her resignation, was taken up by Mr Johnson during his campaign to move into Downing Street.