David Cameron has issued a last-ditch appeal to Boris Johnson not to join the campaign for Britain to leave the European Union.
With the London mayor set to end months of speculation over which side he will back, Mr Cameron said it would be a "wrong step" for Mr Johnson to link up with Ukip leader Nigel Farage and Respect's George Galloway in the "out" camp.
"I would say to Boris what I say to everybody else, which is that we will be safer, we will be stronger, we will be better off inside the EU," the Prime Minister told BBC1's The Andrew Marr Show.
"I think the prospect of linking arms with Nigel Farage and George Galloway and taking a leap into the dark is the wrong step for our country.
"If Boris and if others really care about being able to get things done in our world, then the EU is one of the ways in which we get them done."
Speculation that Mr Johnson is leaning towards the Leave campaign was heightened with the disclosure that he hosted a private dinner with Justice Secretary Michael Gove just days before he declared for out.
It raised the hopes of Leave campaigners that Mr Johnson will now provide the figurehead they have been looking for who can cut through to voters in a way that few other Westminster politicians can.
However Mr Cameron warned that while leaving the EU could create the impression that Britain was reclaiming its own sovereignty, in practice it would be an "illusion".
"If Britain were to leave the EU that might give you a feeling of sovereignty but you have got to ask yourself 'is it real?'," he said.
"Would you have the power to help businesses and make sure they weren't discriminated against in Europe? No you wouldn't. Would you have the power to insist that European countries share with us their border information so we know what terrorists and criminals are doing in Europe? No you wouldn't.
"If suddenly a ban was put on for some bogus health reasons on one of our industries, would you be able to insist that that ban was unpicked? No you wouldn't.
"You have an illusion of sovereignty but you don't have power, you don't have control, you can't get things done."
While Mr Cameron acknowledged that it was possible that Britain could have a trade deal with the EU if it left, he pointed to the example of Canada, which had been negotiating for seven years and still did not have full access to European markets.
"If we leave: seven years, potentially, of uncertainty, and at the end of that process you still can't be certain that our businesses will have full access to the market. So it could cost jobs, it could mean overseas businesses not investing in Britain. It would be a step into the dark," he said.
"The weakness of the Leave campaign is, I think, they forget that even if you leave the EU still exists, it is still on your doorstep."
At a time of great international uncertainty - with the threats of Russian expansionism and Islamic State terrorism - Mr Cameron said there was "strength in numbers".
"In the end this is a hard-headed calculation about what is best for Britain," he said.
"In a world where you have got Putin to the east and Isil-Daesh to the south, how do you stay strong? By sticking with your neighbouring countries, your partners and your friends."
For Labour, shadow foreign secretary Hilary Benn said that it could backfire on Mr Johnson if he chose to declare for the "out" side.
"I'm surprised really because Boris Johnson in the past has written a lot about the importance of staying in the European Union and if he is actually thinking about putting his personal leadership ambitions above the national interest I don't think it's going to do him any good," he told Sky News's Murnaghan programme.
Conservative former minister Liam Fox said he would be surprised if Mr Johnson did not join the Leave campaign.
Asked if it would be a big surprise if the London mayor did not support Brexit, Mr Fox told Murnaghan on Sky News: "Yes I'd be surprised, because I think it is all about this point of sovereignty and the Prime Minister says Britain is now free from ever-closer union."
Eurosceptic Mr Fox said he does not believe Mr Cameron's claim "stands up to legal scrutiny" as there has been no treaty change.
Mr Fox added: "People say: 'Oh, how could you be in the same campaign as George Galloway and others', but the Prime Minister is going to have to link arms with Nicola Sturgeon and Jeremy Corbyn on that side of the argument - not a pretty picture, I have to say."
Mr Farage said he expects Mr Johnson to join the Leave campaign, telling Sky News' Murnaghan show: "I think he will, and 'hurrah' is all I can say to that.
"What again I think a lot of the commentariat in Westminster don't understand is there are literally only five or six people in this referendum whose campaigning, whose presence, can sway the undecideds, and he is one of those half a dozen."