Boris Johnson has said he would find it “very difficult” to vote for Rishi Sunak’s new Brexit deal on Northern Ireland, claiming it is “not about taking back control”.

The former prime minister, speaking for the first time on the new arrangements that replace his Northern Ireland Protocol, said the new Windsor Framework still means having to follow EU laws.

“I’m going to find it very difficult to vote for something like this myself because I believe that we should have done something different. No matter how much plaster came off the ceiling in Brussels,” he said.

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Mr Johnson said he hopes the new deal works but if it doesn’t, the government should have “the guts” to table his controversial Northern Ireland Protocol Bill – which would allow the UK to rip up part of his Brexit agreement without the EU’s permission.

Brussels argued this would breach international law but Mr Johnson insisted the threat of this action “is what brought the EU to negotiate seriously”.

Speaking at the Global Soft Power Summit, the now backbench MP conceded he made mistakes in signing his Northern Ireland Protocol that caused the DUP to walk out of powersharing because of trade barriers in the Irish Sea.

“I thought those checks would not be onerous since there isn’t that much stuff that falls into that category; most of the goods stay in Northern Ireland,” he said.

Muttering, the former prime minister added: “It’s all my fault, I fully accept responsibility.”

Mr Sunak’s new deal aims to reduce those checks while addressing the so-called “democratic deficit” caused by NI being subject to EU trade laws.

But Mr Johnson said he had concerns about the agreement, telling the summit: “I’m conscious I’m not going to be thanked for saying this, but I think it is my job to do so: we must be clear about what is really going on here.

“This is not about the UK taking back control, and although there are easements this is really a version of the solution that was being offered last year to Liz Truss when she was foreign secretary.

“This is the EU graciously unbending to allow us to do what we want to do in our own country, not by our laws but by theirs.”

What is the Windsor Framework?

Mr Sunak reached a deal with the EU on Monday following months of intensive talks aimed at breaking the deadlock over the contentious protocol – an arrangement designed to prevent a hard border on the island of Ireland after Brexit but which effectively placed a customers barrier down the Irish Sea.

The new deal includes:

• Green and red lane trade routes – where goods staying in the UK will use a green lane to avoid customs bureaucracy, while goods moving to the EU will use a red lane
• UK VAT and excise changes will apply in Northern Ireland – British products such as food and drink, trees, plants and seed potatoes will be available in Northern Ireland and pet travel requirements have been removed
• A “landmark” settlement on medicines so drugs approved for use by the UK’s medicines regulator will be automatically available in every pharmacy and hospital in Northern Ireland
• A new “Stormont brake” – to safeguard sovereignty in Northern Ireland. Stormont can stop changes in EU goods laws from applying in Northern Ireland. If the brake is pulled, the UK government will have a veto that will apply permanently

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Mr Sunak has said parliament will get to vote on the deal “at the appropriate time and that vote will be respected”.

One government whip told Sky News the comments from Mr Johnson “will sway very few colleagues” not to support the deal.

While the PM may come up against opposition from some Tory rebels, Labour has promised to back the framework to help get it over the line, saying it is in the national interest to do so.

However, the key group the prime minister has to persuade is Belfast’s Democratic Unionist Party, who pulled out of forming a government in Northern Ireland early last year in protest over the protocol.

Mr Johnson said he hoped DUP leader Sir Jeffrey Donaldson restores powersharing, even as he expressed misgivings about the deal.

This was met with stinging criticism from Naomi Smith, chief executive of internationalist campaign group Best for Britain, who said: “With characteristic shamelessness and self-pity, Boris Johnson has spoken out of both sides of his mouth live on stage, expressing hope for the Northern Ireland settlement while suggesting he is unlikely to vote for it.

“The time for indulging Johnson’s shapeshifting, selfishness and self-promotion at all costs is long past. MPs should back this framework as the first step in cleaning up his Brexit mess.”

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Johnson: ‘Unlikely I’ll do anything big in politics again’

Despite the intervention, Mr Johnson appeared to rule out a return to frontline politics when asked about his future plans, saying: “I think it very, very unlikely that I will need to do anything big in politics again.”

Reflecting on his time at the top of government, he said ministers had not done enough to convince the public of Brexit’s benefits.

“I’ve got to put my hands up for this as much as anybody – we haven’t done enough yet to convince them that it can deliver the change they want to see.

“And I think that they’re particularly dismayed about things like the small boats crossing the Channel, but they also don’t feel the economic change and so we’ve got to break out of the model that we’re in.”

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He also said he wished he had “outbid the Irish” after the pandemic as he called for the government to “get on” with “doing things differently”.

“What I wish we had done is put a big ‘invest here’ sign over Britain as soon as we were out of COVID. As soon as it was remotely credible, I think we should have done something. We should have outbid the Irish,” the former prime minister said.

In a sign he may cause further trouble on the backbenches, he also urged Mr Sunak to slash corporation tax “to Irish levels or lower”, ahead of a planned hike in the tax rate from 19% to 25% in April.