Britain could add nearly 400,000 extra jobs by quitting the EU customs union and forging new trade deals, a pro-Brexit campaign group has claimed.

Analysis by Change Britain said that striking free trade deals with countries including India, China, Japan and Canada could create 387,580 jobs and see almost £20bn ($23.59bn) in exports. The customs union includes the 28 EU member states as well as Turkey, Monaco, San Marino and Andorra.

While they trade freely with one another, they must impose the same tariffs on goods from nations who are not part of the deal, and are banned from signing bilateral trade deals with other countries.

Figures from a 2012 European Commission study suggest six groups have expressed an interest in a trade deal with the UK and Change Britain said it was "reasonable to assume" agreements could also be struck with Japan and the Asean bloc.

The former justice secretary and leading Leave campaigner, Michael Gove, said: "The UK has a prosperous future ahead of us if we leave the EU's customs union and become a beacon of global free trade.

"As we strike new trade deals with the growing economies of the 21st century, it will create hundreds of thousands of jobs right across the country, strengthening communities throughout the UK and ensuring that everyone feels the benefits of economic growth.

"But in order to achieve this we must take back control of our trade policy. Only then can we realise the full potential of this great trading nation," he added.

Change Britain founding supporter and former CBI director general Lord Jones of Birmingham said it was "no surprise" that a number of major economies were interested in striking a deal with the UK given its "rich history as a great trading nation".

He added: "The only way we can make the most of these huge opportunities is to leave the EU's customs union and take back control of our trade policy. This will create hundreds of thousands of new jobs in a range of industries right across the UK. We can then take our place as one of the global champions of free trade."

But Jonathan Portes, a professor of economics and public policy at King's College, rubbished the claims, saying: "Calculating, as Change Britain does, a speculative figure for the number of jobs created by additional exports while ignoring the jobs lost as a result of additional imports is either deeply ignorant or deliberately misleading.

"Whatever your view on the economics of Brexit, Change Britain are again doing their best to distort the debate," he added.

British Prime Minister Theresa May has vowed to set divorce proceedings in motion by triggering Article 50 by March, but the government has been reluctant to divulge further detail other than "Brexit means Brexit."

According to a Sky Data Poll, 48% of people think the government is doing a bad job in Britain's EU departure while 11% think they are doing well. Just 22% think that Britain will get a good deal, while 42% think the opposite.

The poll was conducted between 26 and 28 November 2016 by questioning 1,638 Sky customers.

EU flags Big Ben
Pro-Europe demonstrators protest during a March for Europe protest against the Brexit vote result in London on 3 September 2016 Luke MacGregor/Reuters