Boris Johnson has been accused of trying to back out of his own Brexit deal because he negotiated it too quickly.

No.10 said that the withdrawal agreement (WA) Johnson trumpeted as “oven ready” in the election and signed in January was negotiated “at pace” in “the most challenging possible political circumstances”.

It stressed that Johnson signed up to the deal on the understanding that “ambiguities” would be resolved and new British legislation was needed in case Brussels and London couldn’t find a solution.

But the EU reacted with fury after the government published the Internal Market Bill, which confirmed that the prime minister wants to give the UK the power to renege on the deal and break international law.

And Downing Street’s line prompted Scotland’s first minister Nicola Sturgeon to jibe that Johnson appeared to be justifying its bad faith by saying the UK-EU deal was “signed in a rush”.

“In the General Election it was, according to the PM ‘oven ready’ – now, when they want to jettison it in breach of international law, it was ‘signed in a rush’,” she tweeted.

“What a bunch of incompetent and unscrupulous chancers – and they are trashing the UK’s international reputation.”

Former Tory PM Sir John Major also let rip, putting out a statement that breaking treaties puts at risk “something beyond price that may never be regained.”

European Council president Charles Michel described Johnson’s approach as “unacceptable” and suggested it could lead to the collapse of talks on a long-term trade deal, currently ongoing in London.

Key passages in the Internal Market Bill published on Wednesday confirm Johnson wants to unpick parts of the WA to give ministers sweeping powers to decide when EU rules apply to trade between Northern Ireland and the UK mainland.

The Internal Market Bill, published on Wednesday, gives UK ministers powers to regulate trade in Northern Ireland, with “relevant international law” like the WA “not to be recognised”.

It also gives ministers the power to make regulations to “disapply” an entire section in the WA on state aid rules, which govern how much money a government can give in subsidies to companies or industries.

Sir John Major said: “For generations, Britain’s word – solemnly given – has been accepted by friend and foe. Our signature on any Treaty or Agreement has been sacrosanct.

“Over the last century, as our military strength has dwindled, our word has retained its power. If we lose our reputation for honouring the promises we make, we will have lost something beyond price that may never be regained.”

The plans have already triggered a backlash from Tory MPs and there are growing signs of a civil service walkout over the plans following the resignation of the head of the government’s legal department Sir Jonathan Jones.

David Melding, the shadow counsel general in the Welsh Conservative Party, has also announced he will resign over the move.

Reacting to criticism over the plans, Johnson’s official spokesperson told reporters: “The withdrawal agreement and Northern Ireland protocol aren’t like any other treaty.

“It was agreed at pace in the most challenging possible political circumstances to deliver on a decision by the British people, with the clear overriding purpose of protecting the special circumstances of Northern Ireland.”

No.10 suggested that the “ambiguities” that were left in the agreement were partly due to the time pressure of getting a deal, but the whole point of a joint EU-UK committee was to resolve such outstanding issues.

The new legislation is just a “safety net” in the event of those issues not being resolved, sources said.

But Michel said the WA “has to be applied in full” and warned the prime minister that his approach “does not create the confidence” the EU needs to tie up a long-term trade deal.

The prime minister’s chief aide, Dominic Cummings, has reportedly made getting more freedom over state aid a priority for Brexit negotiations.

On Tuesday, Northern Ireland secretary Brandon Lewis admitted the plans “break international law”.

And after the details were published in the bill on Wednesday, Michel said: “The withdrawal agreement was concluded and ratified by both sides, it has to be applied in full. 

“Breaking international law is not acceptable and does not create the confidence we need to build our future relationship.

State aid is currently one of the main sticking points in ongoing talks on a longer-term trade deal.

Ministers are worried that the WA negotiated by Johnson last year risks giving Brussels control over UK state aid already because of the way it treats Northern Ireland, which will follow EU rules on certain aspects of trade but UK rules on others, to ensure it keeps an invisible border with the Republic.

There is a concern that the EU could interpret these rules broadly and hamstring the UK’s ability to use state aid in the rest of the country, outside Northern Ireland, because it could argue that a mainland company could still be operating in the province. 

The UK is also seeking to seize powers for ministers to unilaterally decide what goods are “at risk” of being sold into the EU via Northern Ireland, and must therefore comply with Brussels tariffs.

Experts believe both of these approaches would put the UK in breach of the WA.

But Downing Street insists that the WA explicitly left many issues on Northern Ireland up in the air, to be hammered out by a so-called “joint committee” of UK and EU officials.

No.10 has said its plans are simply a “safety net” if the joint committee cannot agree.

“We are fully committed to that process,” the spokesperson said, revealing that the UK had accepted a request for a new meeting of the joint committee.

Earlier in the Commons, Johnson said he feared a failure to agree the joint committee could leave the UK open to “extreme and irrational” interpretation of the WA by the EU.

“My job is to uphold the integrity of the UK but also to protect the Northern Irish peace process and the Good Friday Agreement,” he said.

“To do that we need a legal safety net to protect our country against extreme or irrational interpretations of the protocol, which could lead to a border down the Irish Sea in a way that I believe – and I think members around the House believe – would be prejudicial to the interests of the Good Friday Agreement and prejudicial to the interests of peace in our country.

“That has to be our priority.”