Quitting the customs union could allow the UK to forge trade deals creating hundreds of thousands of jobs, a pro-Brexit campaign group has claimed.

The Change Britain analysis suggests that free trade deals with countries which have already indicated an interest in signing an agreement could create over 240,000 jobs.

That figure could rise to almost 400,000 if Japan and the south-east Asian Asean bloc also sign free trade agreements with the UK after it leaves the European Union, the campaign group said.

But campaigners advocating a soft Brexit have warned that exports to the EU could be hit by leaving the customs union, potentially costing British jobs.

The Change Britain analysis suggests that the United States, India, the South American Mercosur group, China, Canada and South Korea have all expressed an interest in a trade deal.

Agreements with those countries would create 240,108 new jobs, the pressure group claimed.

Change Britain said it was also reasonable to assume that deals would be struck with Japan and Asean at some point in the future which, on top of the other agreements, would result in the creation of a total of 387,580 jobs.

Change Britain founding supporter and former CBI director general Lord Jones of Birmingham said: "The UK has a rich history as a great trading nation.

"It is therefore no surprise that a number of major economies have already expressed an interest in striking free trade agreements with us.

"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."

A Department for International Trade spokesman said: "The Government is determined to make a success of Brexit. That's why it's right to take time to work out the best options for the UK's trading relationship after we leave the European Union.

"As we have said before, those decisions have yet to be made."

Labour MP Phil Wilson, speaking on behalf of pro-Europe group Open Britain, said: "These misleading, fantasy figures underline the weakness of the case for leaving the customs union. They take no account of the unavoidable costs that would arise and look at the UK's supposed share from EU trade deals we will not be part of.

"Negotiating alone, rather than in a bloc of 500 million consumers, the UK would be unable to negotiate trade deals of comparable depth to the EU.

"The EU helps increase the UK's global trade links, giving our economy access to over 50 other global markets. We should be looking at ways to protect these benefits, not sacrificing them on the basis of made-up, misleading numbers by anti-EU ideologues."

Liberal Democrat leader Tim Farron said: "Change Britain is continuing the tradition of the Leave campaign by spinning fantasy stories. I can only assume it is competing for the Booker Prize for Fiction. It is ignoring experts and using made-up figures to sell its hard Brexit plan. After the Brexiteers promised £350 million a week for the NHS, no-one should believe a word.

"Leaving the Customs Union would at a stroke deny us access to the economies of 50 countries. As economists, businesses and even the Government's own official figures show, Brexit is a risk to jobs, wages and our wider economy. Leaving the Customs Union would make an already perilous situation worse and would take decades to agree. It would gamble British jobs on foreign trade agreements that are years from being struck."

Asked about the Prime Minister's stance on the Customs Union, a spokeswoman for Theresa May told a Westminster briefing: "The Prime Minister's approach on this has consistently been to focus on how do we get the best deal for the United Kingdom once we leave the European Union.

"As part of looking at that approach, we need to weigh up what the options are. Membership of the Customs Union is not a binary issue. And these are all part of the issues that are being considered."