'Vote Leave' has also pledged to seize back power of the country's borders and finances
As many as 300,000 new jobs will be created if the UK is "liberated from the shackles" of Brussels, Boris Johnson has claimed.
The Out campaigner said a vote to leave the European Union would allow the UK to forge trade deals with developing economies around the world, and insisted many agreements could be enforced immediately after Brexit.
Mr Johnson cited figures from Vote Leave which suggest the EU's inability to secure trade deals with the US, Japan, India, and other trading blocs in South America and Asia have cost the UK more than 284,000 jobs.
The MP said "protectionist forces in Europe" were to blame for the union's failures, adding: "There is a huge world of opportunity and prosperity out there if we take this opportunity to take back control."
At the rally in London's Olympic Park, Vote Leave also pledged to seize back power of the country's borders and finances.
Mr Johnson went on: "The EU goes at the pace of the slowest boat in the convoy and the UK's interests cannot be properly expressed by the Brussels Commission, which has total control of our trade policy."
The former mayor of London's remarks appear to be at odds with fellow Conservative MP and Out campaigner Michael Gove, who said just 24 hours earlier that he cannot guarantee Britons will keep their jobs following Brexit.
But Mr Johnson explained: "What Michael rather brilliantly said was that there might be some job losses among UK Euro MPs."
Vote Leave's claims that the UK will be able to speedily draw up profitable trade agreements in the event of an Out vote have angered David Cameron, who claimed in an interview he will "make them pay".
The British prime minister dismissed the pledge as "nonsense on stilts", and argued such trade arrangements would mean the UK is subject to tariffs which would make goods including shoes, clothes and cars more expensive.
"It's common sense: if you cut yourself off from your biggest market your economy will be poorer - and they know that," Mr Cameron told The Mail On Sunday.
Despite continued infighting ahead of the referendum, now less than three weeks away, Mr Cameron stressed Mr Johnson, Mr Gove and other senior Conservatives who back Brexit "will not be sacked" following the vote.
"We have to bring the party back together. I've always believed in having the big players on the pitch."