The analysis by the Global Futures think tank says that the Prime Minister’s own plans will mean taxpayers lose out by a huge £40bn a year in extra borrowing due to higher trade barriers with the EU.
That equates to a net loss of £615m a week, even after taking account of any savings from quitting the European Union, the policy experts claimed.
The new forecast was accompanied by a new Populus poll showing that even 72% of Leave voters think sums faced by Britain would be “too high a price”.
Some 78% of Leave voters also think such a deal would be “worse than I had hoped for when casting my vote” in the 2016 referendum.
And staying in the Customs Union in a European Economic Area (EEA)-style deal is seen as the best Brexit deal by 37% of those who voted to quit the EU two years ago.
Leaked Whitehall papers earlier this year worked out the cost of various scenarios after Brexit, but none of them included May’s own plans for a ‘bespoke’ deal that kept some trade links albeit with reduced access to EU markets.
The new study, led by former Government civil servant Jonathan Portes, models for the first time May’s plans, based on her Mansion House speech last month.
The ‘Too High A Price’ report states there would be benefits from lower financial contributions and a new trade deal with the US, but costs arising from new customs controls and from reduced migration. It forecasts additional borrowing relative to the status quo of £40 billion in 2033/34.
The Populus poll comes amid a fresh move in the House of Lords to amend the EU (Withdrawal) Bill on Wednesday to keep the UK in the EU Customs Union even after Brexit next March.
When presented with figures that May’s deal will cost Britain £615m a week - rather than save it £350m a week - more than three quarters of Leave voters said that was ‘worse’ than what they’d hoped for.
And when asked which of the various options for Brexit they preferred, the most popular among Leave voters was the EEA - a so-called ‘soft Brexit. The ‘no deal’ scenario of a World Trade Organisation deal was chosen by just 16% of Leave voters.
Portes told HuffPost that the figures showed that talk of a ‘Brexit dividend’ were way off the mark and that the truth was that the PM’s deal would leave Britain poorer by billions of pounds.
“The government claims that its modelling didn’t include Theresa May’s own preferred option of a ‘bespoke deal’. But in fact, based on the Prime Minister’s Mansion House speech, we now have enough information to work out with some confidence what that option looks like - and what, based on the government’s own figures, it would cost.
“Roughly, the price tag would be about £600 million a week – even after taking account of savings from our EU budget contributions.”
“Of course, the £350 million saving from leaving the EU has long since been discredited. But that doesn’t mean that Parliament and voters shouldn’t be able to make decisions about Brexit knowing what the real impact will be on the public finances and the NHS, taking into account both the costs and benefits of different options.”
Global Future CEO Gurnek Bains called for politicians to be open with the public about the trade-offs.
“All decisions have both positive and negative consequences. And this is as true of Brexit, the biggest public policy challenge Britain has faced in a generation, as it is of anything else,” he said.
“But we all have a responsibility to consider both costs and benefits in a clear-sighted way, and to be open with the public about the need to make difficult choices rather than pretending that we can have our cake and eat it.”
A spokesman for the Department for Exiting the European Union (DExEU) said that the analysis was flawed because it was based on provisional plans.
“We are seeking a bold and comprehensive Future Economic Partnership with the EU that is of greater scope than any existing agreement. This includes the greatest possible tariff- and barrier-free trade with our European neighbours,” they said.
“This analysis does not represent that scenario. It is predicated instead on unfinished government analysis of existing scenarios, work which ministers have clearly set out is provisional, internal and ongoing.
“We will deliver on the referendum result by taking back control of our money, our borders and our laws. We will also use the opportunity Brexit presents to build a truly Global Britain, which stands tall in the world, and forges new and ambitious trade deals across the globe.”