Theresa May's former chief of staff has claimed that the Taoiseach's "hardline stance" on the backstop is a threat to peace in Northern Ireland.
Nick Timothy also accused Leo Varadkar of not understanding the Good Friday Agreement.
Writing in yesterday's Daily Telegraph, he said: "The Taoiseach's hardline stance on the backstop risks bringing back a hard border and destabilising the Good Friday Agreement.
"He has backed himself into a corner. He doesn't understand the Good Friday Agreement and he's jeopardising the peace process.
"He's risking a no-deal Brexit and a hard border on the island of Ireland. Yet Leo Varadkar attracts little criticism.
"He continues to defend his Brexit policy with a brave face but, like a poker player who has raised the stakes too high, he must surely feel his stomach churning as he contemplates the consequences of his gamble."
Mr Timothy was joint chief of staff with Fiona Hill. They resigned after the Tories' disastrous 2017 election.
Mrs May had been warned she faced a leadership challenge unless she sacked them.
Mr Timothy said the backstop was supposed to stop a hard border. But, by making the Withdrawal Agreement "unratifiable", it had increased the chances of one. "There can only be one solution: the backstop must go," he declared.
Mr Timothy said the Brexit referendum vote in 2016 was "a nasty reminder" to Dublin that British decisions "can still impinge on Irish sovereignty and that, thanks to geography and history, Ireland's interests remain closely aligned with those of the UK".
Mr Timothy said the Republic imported more from the UK than from any EU country, and the UK was its second biggest European export market.
"Eighty-five per cent of Irish freight trade to Europe goes through British ports.
"A no-deal Brexit would be a catastrophe for Ireland," he said.
"Varadkar therefore had a choice. Recognising our shared interests, he could have argued for a future UK/EU relationship that allowed Britain to leave the single market and customs union while still minimising friction in cross-border trade. He could have continued the work begun by his predecessor, Enda Kenny, to use policy and technology to avoid customs checks along the Northern Irish border."
Instead, the Taoiseach gambled and urged the EU to take the hardest line possible, Mr Timothy said.
"With Varadkar's connivance, Brussels weaponised the Northern Irish border - and with it the peace process - to lock the UK into a customs union and a colonial status in which we would have to follow EU laws.
"And so the 'backstop', the Withdrawal Agreement protocol that binds either Northern Ireland or the whole of the UK to the EU's customs union and many of its laws, was invented.
"It is easy to see why this approach tempted Varadkar and his allies in Brussels. It suits them very well to keep the UK tied to European laws.
"It must also have been hugely enjoyable for a young Taoiseach, and his deputy, Simon Coveney, to lord it over Ireland's former imperial masters." Mr Timothy said he suspected that Mrs May had "liked and wanted the backstop".
He continued: "Dublin's policy was bold, audacious and very nearly successful.
"Varadkar has sought to impose humiliating terms on Ireland's larger and more powerful neighbour.
"Under Theresa May, the UK almost succumbed. But now Boris Johnson is holding firm. When he visits Northern Ireland this week he should not be shy in pointing out who is risking a no-deal outcome and a hard border in Ireland. It is not the United Kingdom, but Leo Varadkar."
Mr Timothy said that unionists represented "a narrow majority of opinion in Northern Ireland and will not simply accept their laws being decided in Brussels and other European capitals, including Dublin".
He noted that the Taoiseach had said the government's Brexit policy would encourage nationalists and liberal unionists to "look more towards a united Ireland".
Mr Timothy said: "These are not the only ways in which Varadkar has played fast and loose with peace in Northern Ireland.
"Lord Trimble, one of the architects of the peace process, explained in a Policy Exchange paper last week how the backstop has 'driven [the Good Friday Agreement] into the ditch'."
The Taoiseach's office was contacted for comment.