Britain will be punished by the EU for leaving because other countries will not want to see it "succeed" alone, Philip Hammond has warned.
The Foreign Secretary delivered the stark message as he insisted negotiations over membership reforms will run "right to the wire" of a crunch summit in Brussels this week.
The comments, in an interview on the BBC's Andrew Marr Show, came as the sides began ramping up their campaigns with just four months to go until the likely referendum date.
Two senior travel industry figures have cautioned that flight prices could rise and tourist safety could be compromised by Brexit.
Mr Hammond said there were still "a lot of moving parts" in the draft deal tabled by European Council president Donald Tusk, but UK had already secured an exemption from "ever-closer union" and a "major breakthrough" on restricting migrant benefits.
Other EU states recognised that Britain needed a "robust deal" in order to stay in.
"Until a few weeks ago people were telling us it was impossible to have any kind of period in which we treated newly arrived migrants differently from people who were already here," Mr Hammond said.
"But the text that is on the table recognises that there can be a period of four years in which people are treated differently. That is a major step forward.
"What we have still got to discuss is what that difference in treatment precisely is ... I don't think that is going to get resolved before Thursday."
Mr Hammond said the negotiations would go "right to the wire, with some of these things only being able to be decided by the heads of state and government on Thursday when they sit down in that room together".
"If we can't get the deal we will carry on talking."
Challenged that the proposals on the table fell short of the Tory manifesto pledge of a four-year ban on migrants claiming in-work benefits, Mr Hammond said: "Let's look at it in the round. There may be areas where we get more than we expected to get and areas where we get slightly less than we expected to get. But it would be absurd not to look at the package in the round.
"Look at all the pluses, all the minuses and weigh the balance."
Asked whether a one-year ban on in-work benefits for migrants would be enough to satisfy his party, Mr Hammond said: "A one-year period would not, definitely not, but we've got four years, a recognition that there can be different treatment for four years.
"Getting agreement that we can treat new arrivals differently for a period of four years is a major breakthrough in challenging, as we have done, one of the sacred cows of European ideology."
Mr Hammond said he feared that if the UK left it would have to forge new relationships with a very different EU.
"What I think I fear and many people in Europe fear is that without Britain Europe would lurch in very much the wrong direction," he said.
"Britain has been an enormously important influence in Europe, an influence for open markets for free trade ...
"I think we would be dealing with a Europe that looked very much less in our image. I think the thing we have to remember is that there is a real fear in Europe that if Britain leaves the contagion will spread.
"People who say we would do a great deal if we left forget that the countries remaining in the EU will be looking over their shoulder at people in their own countries saying, 'Well, if the Brits can do it, why can't we'.
"They will not have an interest in demonstrating that we can succeed outside the EU."
Mr Hammond also refused to be drawn on whether he thought Justice Secretary Michael Gove or London Mayor Boris Johnson would campaign to stay.
"People want to wait and see what the deal is, and clearly there are one or two people whose minds probably are made up but I hope that there are others who are genuinely open to the deal that come back and considering their position on it," he said.
"You will have to ask them. I can't speak for others."