Former Mayor of London Boris Johnson says nearly 300,000 new jobs will be created if Britain escapes from the "shackles" of the European Union (EU).

“If we Vote Leave we will be able to forge bold new trade deals with growing economies around the world. These are deals that the EU has tried and failed to achieve due to protectionist forces in Europe,” said Johnson at a Brexit campaign rally in London on Saturday, adding that, “After we liberate ourselves from the shackles of Brussels we will be able to create hundreds of thousands of new jobs right across the UK.”

Brexit campaigners at the rally pledged to start immediate trade negotiations that would create a boost in employment provided that voters back leaving the 28-nation bloc in the June 23 referendum.

“Predictably the gloomsters want to do down Britain - they claim we are not strong enough to stand on our own two feet. What total tosh. There is a huge world of opportunity and prosperity out there if we take this opportunity to take back control,” Johnson noted.

The campaigners claimed that research in Brussels showed the EU's failure to hammer out trade agreements with the US, Japan, Association of Southeast Asian Nations, India and the South American trade bloc had cost the UK 284,341 jobs.

Britain's Prime Minister David Cameron in an anti-Brexit rally in west London on May 30, 2016. (AFP photo)

Johnson’s remarks came shortly after UK Prime Minister David Cameron accused senior Brexit campaigners including Johnson for "writing checks they know will bounce" after they claimed that leaving the EU would allow the British government to pump £100 million more a week into the UK’s National Health Service (NHS) by 2020.

In a tweet on Saturday morning, Cameron said the Vote Leave campaign was “writing checks they know will bounce. Economists say there’ll be a profound shock if we leave the EU. That means there will be less money - not more. It’s also why so many doctors and nurses support remaining in the EU.”

Britain will vote on June 23 on whether or not it should stay in the EU. The decision has far-reaching consequences for both the country and the bloc.

Membership of the European Union has been a controversial issue in the UK since the country joined the then European Economic Community in 1973.

Those in favor of remaining in the bloc argue that leaving it would risk the UK's prosperity, diminish its influence over world affairs, and result in trade barriers between the UK and the EU.

On the spectrum, Britons who favor withdrawal believe that outside the bloc, the UK would be better off in conducting its own trade negotiations, better able to control immigration and free from what they believe to be excessive EU regulations and bureaucracy.