Boris Johnson’s post-Brexit trade deal with the EU leaves workers’ rights and environmental protections at risk of erosion and will slow the economic recovery, an early analysis has warned.

The IPPR think tank’s assessment, published on Sunday, says that the bar for proof of breaches of the “level playing field” to safeguard the issues is so high that it will be rarely enforced.

The Prime Minister has insisted that the UK will not regress on the issues and instead has vowed to use the “legislative and regulatory freedoms to deliver for people who felt left behind”.

But as Tory Eurosceptics pore over the 1,246-page treaty to see if they can give their backing, the IPPR added to criticism of the deal from the fishing industry.

Marley Morris, a director focusing on trade and EU relations, warned that the commitment in the deal not to decrease current standards in a bid to gain an unfair competitive advantage is “considerably weaker than expected” and sets “a very high bar for proof”.

“Given it is notoriously difficult to prove that any lowering of protections affects trade or investment, the deal is unlikely to prevent the UK Government from weakening EU-derived labour and environmental policies if it so chooses,” his report says.

He later added: “This leaves protections for workers, climate and the environment at serious risk of being eroded.”

The IPPR also said that by the UK having “watered down” the level playing field requirements to secure “only limited benefits in market access” there will be a blow to trade.

There will likely be disruption to trade flows, including at the border, in the short-term while barriers to trading with the nation’s biggest partner will “likely lead to slower growth and a more prolonged economic recovery” from the coronavirus pandemic, the analysis says.

“This thin deal is better than no deal at all, but still creates major trade barriers with our closest neighbour, which will inhibit growth and slow the economic recovery,” Mr Morris added.

A Government spokeswoman responded that ministers are “fully committed” to upholding the “high standards” agreed in the trade deal.

A hasty analysis of the treaty secured on Christmas Eve began in earnest when it was published in full on Boxing Day – less than a week before its implementation.

In his first interview since brokering the agreement, Mr Johnson denied the UK would regress on workers’ rights and environmental standards, two issues both sides have committed to uphold in the deal.

Mr Johnson told the Sunday Telegraph: “All that’s really saying is the UK won’t immediately send children up chimneys or pour raw sewage all over its beaches. We’re not going to regress, and you’d expect that.”

He said that Chancellor Rishi Sunak is “doing a big exercise” on business taxes and regulation alongside a “great Government effort” for new plans for after the transition period ends on December 31.

But Mr Johnson said it “perhaps would not have been fruitful” to discuss them publicly during negotiations, as he listed animal welfare regulations, data and chemicals, alongside existing plans to establish low-tax freeports.

The Prime Minister did, however, acknowledge that the treaty “perhaps does not go as far as we would like” over access to EU markets for financial services.

But Mr Sunak said the nation will be able to “do things a bit differently” now, referencing new opportunities for the sector, and said the deal should leave those anxious about the financial impact “enormously reassured”.

“I actually think this deal can represent an enormously unifying moment for our country and bring people together after the divisions of the past few years,” he told broadcasters.

The chief executive of the National Federation of Fishermen’s Organisation (NFFO), Barrie Deas, accused Mr Johnson of having “bottled it” on fishing quotas to secure only “a fraction of what the UK has a right to under international law”.

Mr Deas said the Prime Minister had “sacrificed” fishing to other priorities, with the subject proving to be an enduring sticking point during negotiations as they raced to get a deal by the end of the transition period on December 31.

The share of fish in British waters that the UK can catch will rise from about half now to two-thirds by the end of the five-and-a-half-year transition.

Mr Johnson has said that, although he accepts that “the devil is in the detail” of the deal, he believes that it will stand up to inspection from the European Research Group (ERG) of Brexiteers.

The group has convened a self-styled “star chamber” of lawyers led by veteran Eurosceptic MP Sir Bill Cash to examine the full text ahead of a Commons vote on December 30.

The senior Conservative backbencher said “sovereignty is the key issue” as his team analysed the small print of the deal, with their verdict expected to be announced on Tuesday.

But there were indications Brexit hardliners were preparing to support the deal, despite being angered by the little time they have to debate it.

The agreement will almost certainly be passed by Parliament, with Labour supporting what it criticises as a “thin” treaty as the alternative would be a chaotic no-deal situation on January 1.

Ian Blackford, the SNP’s leader in Westminster, said his party would vote against the “unforgivable act of economic vandalism and gross stupidity”.

Meanwhile, International Trade Secretary Liz Truss said she expects to sign a continuity trade agreement with Turkey this week, a move that was not possible until the deal with the EU was struck because Ankara is in a customs union with the bloc.

EU ambassadors will meet on Monday to determine how the deal covering £660 billion of trade can be provisionally approved ahead of formal ratification by the European Parliament.

On Sunday, an EU diplomat said: “The EU will take the decision to provisionally apply the EU-UK agreement by written procedure. EU ambassadors are expected to initiate the process tomorrow.”