Boris Johnson is “refusing to be honest” about the likely damage from his Brexit plans, which risk a “levelling down” – rather than levelling up – of the economy, experts warn today.

A damning report warns the prime minister is on course to strike only “the barest of bare bone deals” with the EU by his deadline of the end of 2020, because of the strict red lines he has drawn.

But, it says, his government is failing to be upfront about the inevitable “trade-offs” to come and the adverse consequences for businesses, citizens and taxpayers.

“Ministers have refused to be honest with the public about the choices they will make particularly in terms of economic impact, the increase in red tape, and growth in the size of the state,” the UK in a Changing Europe think tank concludes.

The verdict comes after Mr Johnson sought to rebrand an effective no-deal Brexit in December as an agreement “like Australia’s” – even though Canberra has no agreement.

The claim, in his first big post-election speech, is widely seen as softening up the public for damaging World Trade Organisation trading terms, something he insisted there was “absolutely zero” risk of before polling day.

Now the UK in a Changing Europe has explored the “mammoth” challenges ahead for everything from trade, the environment and immigration to security, the Union and regional inequality.

On trade, it warns Mr Johnson’s refusal to extend the transition period into 2021, and determination for the UK to make its own rules, makes anything other than a skeleton deal “hard to agree”.

“It will mean new trade friction for agriculture and manufacturing; and significant new barriers for service exports,” the report concludes.

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“A bare bones trade agreement that just eliminates tariffs could be costly for manufacturing to live with given how tightly entwined manufacturing value chains are across the UK and the EU.”

Next month’s Budget is meant to set out how the government will ‘level up’ the UK after leaving the EU, but the report warns: “Brexit risks ‘unlevelling’ us still further.

“While the UK suffers some of the largest interregional inequalities in the developed world, these might be exacerbated if, as some economists expect, the more prosperous parts of the country prove to be less affected by and/or more resilient to, any economic impacts of Brexit.”

On security, the report warns the UK will inevitably be shut out of vital crime-fighting tools including the European Arrest Warrant, Schengen Information System and European Criminal Record Information System.

“Whatever can be agreed on security will mean less access than now to the EU’s law enforcement and judicial cooperation tools,” it states.

“Getting Brexit done, in the sense of leaving the European Union was, in a way, the easy bit,” said Professor Anand Menon, the think tank’s director.