Boris Johnson broke his silence on the UK's new Brexit deal on Thursday by saying he would find it “very difficult” to support the agreement.
The former prime minister used a speech in London to say the deal struck by his successor Rishi Sunak did not “take back control” from EU law.
He did not commit to voting for or against the deal on Northern Ireland, but conceded that the “political momentum” was with Mr Sunak.
The Windsor Framework announced on Monday has rewritten the Northern Ireland trade pact negotiated by Mr Johnson in 2019.
In return for simpler trading rules, Britain conceded that the European Court of Justice will still have the final say over EU laws applied in Northern Ireland.
While some ardent Brexiteers have swung behind Mr Sunak, Northern Ireland's unionists are still pondering their position, and Mr Johnson's intervention had been keenly awaited.
Mr Johnson said at a conference in London that the EU's continued influence would hold Britain back from exploiting its post-Brexit freedoms.
He said the threat he made to unilaterally set aside the UK-EU treaties should be kept available if needed, after Mr Sunak agreed to drop the contentious Northern Ireland Protocol Bill.
“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”, Mr Johnson said.
“This is not about the UK taking back control. 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.”
Whether any Conservative MPs would follow Mr Johnson's lead in any vote is unclear, but his stance takes some of the shine off what has the makings of a political win for Mr Sunak.
Amid persistent speculation that he will attempt a comeback, he declared “purely for accuracy” that the Conservative Party's poll rating had declined since he left office.
However, he conceded that his 2019 Northern Ireland deal had led to more onerous trade checks than he predicted at the time.
UK and EU strike Brexit deal on Northern Ireland — in pictures
After few in the audience expressed support for his position, he said he “obviously needs to do a better job of explaining and supporting and defending Brexit.”
The existing Brexit deal led to concerns that Northern Ireland's ties with the British mainland were being loosened, angering unionists.
The revised agreement gives the Northern Ireland Assembly what Britain calls a veto over new EU legislation, which Brussels says would only be used in rare circumstances.
Mr Johnson said he hoped the region's Democratic Unionist Party would return to power-sharing under the 1998 Good Friday Agreement, after pulling out in protest at the checks.
The US and leading European governments have praised the new agreement, which comes shortly before the 25th anniversary of the Good Friday peace deal.