The Change Britain group said that the option - which it describes as “clean Brexit” - is likely to deliver annual savings of almost £10.4 billion from contributions to the EU budget and £1.2 billion from scrapping “burdensome” regulations, while allowing the UK to forge new trade deals worth £12.3 billion.
The group said its estimate was “very conservative” and that the benefits of withdrawal from the single market and customs union could be as much as £38.6 billion a year.
Even the lowest forecast within its range of likely outcomes was a boost of £20 billion. But the figure does not factor in the possibility of large-scale loss of exports to the remaining 27 EU nations, which advocates of a “soft Brexit” argue could happen if the UK faces tariff and non-tariff barriers to trade as a result of leaving the single market.
Britain exported around £220 billion of goods and services to the EU in 2015, while imports from the EU totalled around £290 billion, the Press Association reported.
Change Britain said that the biggest prize on offer was in potential trade agreements outside the EU which Britain could strike if it left the customs union, which requires it to take part only in deals negotiated by the European Commission.
Depending on how many deals the UK secures, GDP could be boosted by between £8.5 billion and £19.8 billion, said Change Britain.
Biggest benefits could come from a trade deal with the Asean group of south-east Asian economies, which could increase GDP by as much as £4.3 billion a year, followed by the USA (£3.8bn), Japan (£3.2bn), South Korea (£3.2bn), Canada (£1.9bn), the Mercosur zone in South America (£1.8bn), India (£1.5bn) and China (£200 million), said the group.
Leaving the single market would allow Britain to scrap 59 of the 100 most burdensome regulations on business, saving more than £4 billion, said Change Britain.
However, as the Government has indicated that it will preserve some of the rules - including protections for workers - the group estimated the likely saving at around £1.2 billion.
And it said that the Government’s indication that it will continue support to farmers and other sectors currently subsidised by the EU means that the total saving from ending contributions to Brussels budgets will be around £10.4 billion, rather than the £14.7 billion which the UK currently pays.
Conservative MP for Dover & Deal and Change Britain supporter Charlie Elphicke said: “This report makes it clear that there are huge opportunities to be had from making a success of Brexit. Leaving the EU fully means we won’t have to pay billions to bloated Brussels bureaucrats. It means we will be able to take back control of our borders and end uncontrolled EU immigration.
“What’s more, we’ll be able to spend money on the things that matter to the British people. We can build a modern Britain that puts the hard-working classes first and makes it easier for British businesses to trade in the world and succeed.
“This is what people voted for in the referendum. Now everyone, Remainer or Leaver, has a duty to get on with the job of delivering a brighter future for our land.”
Yet in November, the OBR worked out that the economy would be £122bn worse off by 2020.
Lower growth, higher inflation and a weak pound - all caused by business uncertainty after the Brexit vote - have created a £58.7bn hit to the public finances, the watchdog said.