Ex-minister Jacob Rees-Mogg accuses Rishi Sunak of failing to get support from DUP and Tory MPs.
No 10 has insisted it is engaging with DUP and Conservative MPs as the prime minister seeks agreement with the EU on post-Brexit rules in Northern Ireland.
Rishi Sunak is trying to resolve issues with the Northern Ireland Protocol, introduced after the UK left the EU.
However, Conservative ex-minister Jacob Rees-Mogg has warned his efforts are doomed unless he gets backing from unionist parties and Brexit MPs.
MPs concerned about the PM’s plans are due to meet later on Tuesday.
Earlier, the prime minister told his cabinet ministers that attempts to reach a deal were focused on safeguarding Northern Ireland’s place in the UK, protecting the Good Friday Agreement and ensuring the free flow of trade in the UK internal market.
The current rules, known as the Northern Ireland Protocol, were negotiated by Boris Johnson and came into force in 2021.
They introduced post-Brexit checks on goods sent from Great Britain to Northern Ireland, to get round the need for checks at the UK’s border with the Republic of Ireland.
The rules have proved highly unpopular among unionists in Northern Ireland, and soured relations between the UK and EU.
In protest at the rules, the DUP (Democratic Unionist Party) boycotted power-sharing in Northern Ireland, meaning it has been without a functioning devolved government since February of last year.
A majority of members of the Stormont assembly are in favour of the protocol in some form remaining in place.
Sinn Féin, the Alliance Party and the SDLP have said improvements to the protocol are needed to ease its implementation.
British and European negotiators have been locked in talks for over a year to secure changes that will satisfy by business groups and politicians.
‘Life doesn’t work like that’
On his Conservative Home podcast, Mr Rees-Mogg argued there was “no point” agreeing a deal which does not have the support of the DUP.
“I don’t know why so much political capital has been spent on something without getting the DUP and the ERG (European Research Group of Conservative MPs) onside first,” he said.
He likened Mr Sunak’s approach to that of former Prime Minister Theresa May who, he said, had presented a policy in the hope that people would “conveniently fall in behind” it, he said.
“Life doesn’t work like that. It’s important to get support for it first before you finalise the details and that doesn’t seem to have been done here.”
He urged Mr Sunak to instead pass the Northern Ireland Protocol Bill, which would give the UK government the power to rip up parts of the current arrangement with the EU.
His comments echo those of former-Prime Minister Boris Johnson, who over the weekend urged his successor not to ditch the bill.
Home Secretary Suella Braverman also appeared to indicate her support, describing the legislation as one of the “biggest tools” to solving issues over trade in the Irish Sea.
On Tuesday, the prime minister’s spokesman told reporters “intensive negotiations continue” between the UK and EU but added that unresolved issues and “long-lasting challenges” needed to be addressed.
He rejected suggestions the DUP and Conservative Brexiter MPs had not been sufficiently involved.
“We have been speaking to relative parties at the appropriate times throughout this process,” he said adding that “engagement will continue as we continue to negotiate”.
Foreign Secretary James Cleverly and Northern Ireland Secretary Chris Heaton-Harris are due to hold fresh talks over video link with European Commission vice-president Maros Sefcovic on Tuesday afternoon.
Last week Mr Sunak travelled to Belfast to meet politicians in Northern Ireland and then Germany, where he met European Commission President Ursula von der Leyen, raising speculation that an agreement was imminent.
However, DUP chief whip Sammy Wilson told Sky News he did not expect a deal to be reached this week.
“If a deal is agreed which still keeps us in the EU single market, as ministers in the Northern Ireland Assembly we would be required by law to implement that deal and we are not going to do that because we believe such an arrangement is designed to take us out of the United Kingdom.
“We are British and we expect to be governed by British law, not Brussels law. We would certainly not collaborate in administering Brussels law in our part of the United Kingdom.”