Boris Johnson has told the EU the backstop plan for the Irish border must be scrapped because it is « unviable » and « anti-democratic ».
In a letter to European Council President Donald Tusk, the PM said the backstop – which aims to avoid a hard border – risked undermining the Northern Irish peace process.
If the plan were removed, Mr Johnson claimed a Brexit deal would be passed by Parliament.
Brussels has not yet responded.
However, the EU has consistently insisted the backstop must remain part of the withdrawal agreement and cannot be changed.
In a phone conversation with Mr Johnson on Monday evening, Irish Taoiseach (prime minister) Leo Varadkar reiterated that the agreement – negotiated by former PM Theresa May but rejected by Parliament three times – could not be reopened.
Mr Johnson’s four-page letter to Mr Tusk comes ahead of meetings this week with German Chancellor Angela Merkel and French President Emmanuel Macron.
The border is a matter of great political, security and diplomatic sensitivity, and both the UK and EU agree that whatever happens after Brexit there should be no new physical checks or infrastructure at the frontier.
The backstop is a position of last resort to guarantee that, but if implemented, it would see Northern Ireland stay aligned to some rules of the EU single market.
It would also involve a temporary single customs territory, effectively keeping the whole of the UK in the EU customs union.
In his letter, Mr Johnson described the arrangement as « inconsistent with the sovereignty of the UK » and insisted it could not form part of a withdrawal agreement.
He also warned that it risked « weakening the delicate balance » of the Good Friday peace agreement because unionist parties like the DUP are so unhappy with it.
The prime minister called for « flexible and creative solutions » and « alternative arrangements » – based on technology – to avoid a hard border.
He said the backstop should be replaced with a commitment to put in place such arrangements as far as possible before the end of the transition period – currently the end of 2020 under Mrs May’s deal.
If they were not in place by the end of the transition period, Mr Johnson said the UK was « ready to look constructively and flexibly at what commitments might help ».
« Time is very short. But the UK is ready to move quickly, and, given the degree of common ground already, I hope the EU will be ready to do likewise, » he wrote.
« I am equally confident that Parliament would be able to act rapidly if we were able to reach a satisfactory agreement which did not contain the backstop. »
View from the EU
The prime minister’s letter shows that the UK is not committed to a fully worked up – « legally operable » – insurance policy for the Irish border to be contained in the Brexit divorce treaty.
Instead, Mr Johnson has proposed using the post-Brexit transition period to search for alternative ways of keeping the border open.
If they can’t be found, the UK is committing to having a fall-back option. Yes… something you might call « a backstop ».
A lot of this had been telegraphed to the EU in advance by the prime minister and his advisers but it feels different now it’s on paper.
European diplomats wonder whether they are being asked to sign up to something that is too vague, too difficult to achieve, or just too hard for them to accept.
Privately, they fall into three camps: those closely involved with the drafting of the backstop who feel the proposal crosses the EU’s red lines; those who think it is a stunt rather than a genuine offer; and some who believe it provides an opening to break the Brexit stalemate.
However, Labour pointed out that Mr Johnson actually voted for Mrs May’s deal – including the backstop – when it came before Parliament for a third time in March.
Mr Johnson said at the time he was only doing so because he had reached the « sad conclusion » it was the only way to ensure the UK actually left the EU.
« Whichever Brexit outcome he pursues, whether it’s a disastrous no deal or this fantasyland wish list, Boris Johnson clearly has no qualms about putting jobs, rights, prosperity or peace in Northern Ireland at risk, » shadow Northern Ireland secretary Tony Lloyd said.
Meanwhile, Labour leader Jeremy Corbyn is calling on the government to publish all documents on the impact of a no-deal Brexit, after a leak at the weekend suggested there could be significant disruption to supplies of food and medicine.
Michael Gove, the cabinet minister responsible for no-deal planning, has said the information in that leaked dossier was old and Brexit planning had accelerated since Mr Johnson became prime minister.
However, Mr Corbyn said: « If the government wants to be believed that it doesn’t represent the real impact, it must publish its most recent assessments today in full. »
A government spokesperson said up-to-date information on what businesses and members of the public needed to do to prepare for the UK’s departure from the EU was available on the government’s website.