By Nicholas Winning 

LONDON--Negotiations on the U.K.'s demands for changes to its relationship with the European Union are likely to go to the wire and some issues won't be decided until the bloc's leaders meet at a summit in Brussels on Thursday, U.K. Foreign Secretary Philip Hammond said Sunday.

In an interview with the British Broadcasting Corp., Mr. Hammond said the EU had agreed to the principle that the U.K. could have a special regime for new European migrants--such as restrictions on welfare entitlements--for four years. The move signals a major step forward but Britain would have to work with other member states to determine how it would work.

"What we have still got to discuss is what that difference in treatment precisely is," he said. "I don't think that is going to get resolved before Thursday. That will be on the table when the prime minister is sitting in the European Council on Thursday."

Prime Minister David Cameron has promised to redraw Britain's relationship with the EU and then hold a national referendum on membership of the bloc by the end of 2017. The referendum could come as soon as June if he manages to secure agreement on his proposals at the two-day European Council summit starting this week.

Mr. Cameron's plans for EU migrants to have to wait four years before they are eligible for in-work welfare benefits in Britain is the most contentious of his demands. Mr. Hammond said it wouldn't be enough for those new migrants to be denied benefits for only one year.

On Monday, foreign ministers from the so-called Visegrad group, which represents Poland, Hungary, Slovakia and the Czech Republic, are set to meet in Prague to discuss a common position ahead of the leaders summit.

The four countries are among those most concerned about the proposals to restrict benefits for migrants, with hundreds of thousands of their citizens having moved to the U.K. in recent years.

However, in an interview at the Munich Security Conference this weekend, Slovak Foreign Minister Miroslav Lajcak said the proposals as they stood didn't cross his government's red lines.

He said his government wanted to make sure that the in-work restrictions don't affect current EU workers in Britain, that there is no permanent discrimination against EU nationals and that only child benefits would be affected by a proposal to index welfare payments to the standard of living of the countries where a worker's dependents actually lived. That could allow the U.K. to reduce its child benefits bill.

"I think there's a fair chance we have a deal at the summit," Mr. Lajcak said. "We have always expressed our understanding of the British" position.

Mr. Hammond said that if Britain cannot get the right deal on its four key demands on competitiveness, the relationship between EU member states with the euro and those outside the eurozone, national sovereignty and access to welfare benefits, "we will carry on talking."

"Our European partners understand that we have to have a robust deal in each of those areas if the British people are to vote to remain inside the European Union," he said.

Mr. Hammond said it was already clear that Britain would get a statement that it was outside the EU treaty commitment to "ever closer union" and there would be a framework for the relationship between eurozone and non-eurozone countries within the EU.

"We already seeing the shape of a deal but there's still a lot of moving parts yet over the next few days," he said.

Laurence Norman in Munich contributed to this article.

Write to Nicholas Winning at nick.winning@wsj.com

(END) Dow Jones Newswires

February 14, 2016 10:55 ET (15:55 GMT)

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