His remarks will come as a timely boost to campaigners hoping to get Britain out of the 28-nation bloc, with opinion polls showing a small majority of Britons now favour severing ties with Brussels.
The Eurosceptic Mayor of London, who is hotly tipped to become leader of the Tory party when David Cameron steps down in 2020, made the remarks during a trip to the Far East.
Mr Johnson is backing David Cameron’s efforts to secure meaningful reform from European leaders, but has previously warned that Britain must be prepared to leave the bloc if it is not prepared to change.
Former M&S boss Stuart Rose at the launch of the pro-Brussels campaign
“We want, in an ideal world, to stay in a reformed European Union but I think the price of getting out is lower than it’s ever been. It’s better for us to stay in, but to stay in a reformed EU. That’s where I am.”
The Tory big hitter also refused to quash speculation that he could ultimately end up leading the cross-party Vote Leave campaign, which launched on Friday.
The influential group, which is being backed by Britain’s most successful businessmen, will have a £20million war chest to campaign for a Brexit.
The campaign to keep Britain in the EU launched today
I think the price of getting out is lower than it’s ever been
Mr Johnson did not dismiss the possibility when asked, merely stating: “You are now mounting hypothesis upon hypothesis.”
His remarks came on the same day the Government launched its own campaign to keep Britain in the EU.
The movement, called Britain Stronger in Europe, tried to position itself as the ‘patriotic’ option by claiming it is best for the UK to remain within the 28-nation bloc.
Young volunteers at the launch of the Britain Stronger in Europe campaign
At a launch party for the campaign, former police chief Sir Hugh Orde also warned that Britain could be viewed as a safe haven by international criminals if the country voted to end its relationship with Brussels.
He said: “If I was a villain somewhere else in Europe and I was escaping justice, I would be coming here because it’s going to take a lot longer to get you back.
“Citizens in this country would be put at risk as a consequence. That’s not scaremongering, it’s simply the fact.”
Dismissing Lord Rose’s argument, he said: “He argues that in order to trade we have to have free movement of people.
“Nowhere else in the world is that the case. To think that UK manufactured cars would face tariffs when we import hundreds of thousands more cars from Germany shows utter detachment from the realities of trade.”
The Prime Minister will have lunch with European Commission president Jean-Claude Juncker on Thursday to discuss the latest progress on his demands for EU reform.
His demands have been met with a frosty reception by many EU leaders, who have warned that they would require treaty changes they are not prepared to sanction.