The costs of leaving the European Union are “lower then ever”, Boris Johnson has said, contradicting warnings issued by business leaders hours earlier at the launch of what is likely to be the official pro-Europe campaign.

The London mayor and MP for Uxbridge and South Ruislip has flirted with throwing his weight behind the campaign to leave the European Union in the upcoming referendum.

Speaking to the BBC while on a trip to Japan, Johnson said: “I think I am exactly where the prime minister is and, I think, actually a huge number of the proportion of the British public.

“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.”

Johnson made the comments after the official launch of the pro-EU Britain Stronger in Europe campaign led by the former Marks & Spencer executive chairman Stuart Rose. Rose issued a warning about the economic impact of an exit, claiming three million jobs would be put at risk if the UK quit the EU.

Also speaking at the launch, the former head of the Association of Chief Police Officers (Acpo), Sir Hugh Orde, said that fugitives across Europe would flock to the UK as a safe haven if it left the European Union because a series of laws and extradition agreements would be ripped up.

Although the Electoral Commission hasn’t yet named the official campaigns on either side of the EU debate, the Britain Stronger in Europe campaign is likely to be designated as the official “remain” campaign.

The official campaigns on either side will benefit from an increased spending limit of £7m during the campaign period, public grants of up to £600,000, and access to the electoral register.

Although Cameron’s official position is that he will not take a stance on the referendum until he has completed his efforts to renegotiate Britain’s membership of the EU, he is widely expected to back the campaign to remain.

Speaking on Sunday, Ukip leader Nigel Farage said he would welcome it if Johnson decided to back the EU out campaign. “We might just get him,” he said. “We might just get him and he is a recognisable figure. That would be good news.”