David Cameron has warned Boris Johnson against "linking arms with George Galloway and Nigel Farage" and backing the campaign for Britain to leave the European Union.
Johnson is expected to clarify his position on the EU referendum, which has been announced for 23 June, on Sunday, and speculation is mounting that he intends to defy the prime minister by backing a Leave vote.
Cameron, appearing on The Andrew Marr Show on Sunday, in his first in-depth interview since he announced his EU renegotiation and the date of the referendum, flashed a warning to the mayor of London that backing Leave would be a "leap in the dark".
"I'd say to Boris what I'd say to everyone else, which is we will be safer, we will be stronger, we will be better off inside the EU," said the prime minister. "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, 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. We're members of NATO, we're members of the UN, we're members of the IMF."
Johnson is the last major figure in the cabinet who is yet to announce his position on the EU referendum. The Mail on Sunday reported that he had held talks with Michael Gove, who backed an EU exit on Saturday, about the referendum and Johnson's economic adviser, Dr Gerard Lyons, spent Sunday morning tweeting a case for leaving the EU.
Johnson's sister, Rachel, told Sky News that the London mayor will announce which way he intends to vote in his Telegraph column, which will be available online from 10pm on Sunday night.
Cameron said 23 of the 29 members of this cabinet have backed him on the referendum question, and added every member of his cabinet thought he had achieved a good deal on Friday.
He said he was "sad" some of his friends had backed the Leave campaign, but warned Johnson that leaving the EU will leave Britain less able to "fix stuff".
"I care about Britain being able to fix stuff. Whether it's stopping pirates off the African coast, whether it's closing down illegal migration routes and closing down smugglers, whether it's standing up to Vladimir Putin with sanctions, whether it's the sanctions we put in place to get Iran to abandon its nuclear program.
"Having that seat at the table in the EU, just as being a member of NATO, is a vital way that we project our values, our power, and our influence on the world. I don't say this for any exercise of national vanity, this is about our national interest."
However, he said if the people of Britain did vote to leave the EU, he would continue as prime minister and do "everything in my power" to implement that position, despite disagreeing with it.
"Britain, this amazing country, the fifth largest economy in the world, we can succeed whatever we do," said Cameron. "But if you ask me, having sat here as prime minister for six years, how we'd be safer, stronger, better off in or out, I'm absolutely clear we'll be better off in and I'm going to take that message around the country for the next four months.
"If the British people make a contrary decision I will do everything in my power to implement as best I can, but I'm absolutely clear, after the years I've spent in this job, that this is the right thing for Britain to do."
UPDATE
Boris Johnson has confirmed he will campaign for Britain to leave the EU.
"The last thing I wanted was to go against David Cameron or the government, but after a great deal of heartache, I don’t think there’s anything else I can do. I will be advocating vote Leave," Johnson told press on Sunday evening.