Boris Johnson has suggested he may not vote for Rishi Sunak’s new Brexit deal for Northern Ireland, criticising it as “not about the UK taking back control”.

The Prime Minister’s chief political rival said on Thursday he would find it “very difficult” to support the Windsor agreement despite claiming some credit for the new pact being brokered.

In his first major speech in the UK since being ousted from office, the former prime minister urged his successor not to slash corporation tax to “outbid the Irish”, and to “do things differently” to not betray the ideals of Brexit.

Mr Sunak managed to secure a new deal from the European Union to replace Mr Johnson’s much-criticised Northern Ireland Protocol that had seen the Democratic Unionist Party (DUP) walking out of powersharing in Stormont.

Mr Johnson conceded people “want to move on” and sign the deal, adding that “they don’t want any more ructions and I get that, I’ve got to be realistic about it”.

“When I look at the deal we have now I, of course, have mixed feelings.

“I’m conscious of where the political momentum is and people’s deep desire just to get on,” he added.

I’m going to find it very difficult to vote for something like this myself

However, he raised numerous concerns about the Windsor Framework at a global soft power summit before telling the audience: “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.”

Under the deal “British genome edited tomatoes could not go into the making of a cheese and tomato sandwich in Northern Ireland, which is a matter of great regret”, Mr Johnson said.

He also challenged Mr Sunak to cut corporation tax to “Irish levels or lower”, arguing there is “no point in just emulating the high tax high spend low growth European model”.

Mr Johnson said he has “no doubt at all” that his controversial Northern Ireland Protocol Bill threatening to override international law “is what brought the EU to negotiate seriously”.

Mr Sunak scrapped the Bill after brokering the new deal, but Mr Johnson urged him to have the “guts” to bring it back if the agreement does not succeed.

Mr Johnson added: “I’m going to find it very difficult to vote for something like this myself, because I believed we should’ve done something very different. No matter how much plaster came off the ceiling in Brussels.”

During the speech for the Brand Finance consultancy, Mr Johnson argued Northern Ireland would remain under EU law, with Brussels still making regulations, but “with what they hope will be lighter bureaucracy” under Mr Sunak’s deal.

He said he is “particularly concerned” about goods going through manufacturing in Northern Ireland from Great Britain undergoing checks in some instances which he argued seems “pretty crazy”.

But in a moment approaching humility, Mr Johnson conceded he made mistakes with his protocol.

“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, he added: “It’s all my fault, I fully accept responsibility.”

Earlier in the day, Downing Street said that some people are “misunderstanding” the Windsor Framework signed with the EU’s Ursula von der Leyen on Monday.

Mr Sunak is almost certain to win a Commons vote on the deal because he has Labour’s support, but he is not finding much in the way of vocal criticism in the Tory party either.

Brexit hardliners on the Conservative European Research Group (ERG) are analysing the deal to see if they will support it, while the DUP is having its own debate.

Responding to Mr Johnson’s comments, a Downing Street spokesman said: “The PM understands that people will have questions and opinions relating to the Windsor Framework but he believes this is the best deal for the people of Northern Ireland and for businesses in Northern Ireland.”