LONDON: British Prime Minister David Cameron will demand stronger powers to curb immigration from the European Union when he meets European Council President Donald Tusk on Sunday, a senior British government source said. Cameron will insist that a proposed 'emergency brake' to deny benefit payments to working migrants needs significant strengthening, and argue it must be triggered immediately after Britain's referendum on EU membership, the source said. Cameron has promised to reform Britain's ties to the EU and hold a public vote on EU membership before the end of 2017. He is hopeful of reaching a reform deal at a summit next month, with a view to holding the referendum as early as possible. Reuters reported on Thursday that the EU was offering the emergency brake for up to four years as an alternative to Cameron's proposal to bar EU immigrants from in-work benefits for at least their first four years in Britain. The Prime Minister, who wants Britain to stay in a reformed EU but has not ruled out campaigning for an exit if he doesn't get what he wants, will meet with Tusk later on Sunday to discuss how such a brake could work. But Cameron believes the EU's counteroffer should be applicable for as long as is necessary to solve the underlying problem and only treated as a stop-gap measure before a permanent mechanism can be established, the source said. Finding a way to curb migration has proved to be the most troublesome element of Cameron's renegotiation, which also seeks to address voter concerns over competitiveness and sovereignty. Leaders of several of the EU's 28 member countries have said they would block any measure that discriminated against their citizens or undermined the core EU principle of free movement of people.