WARSAW (Reuters) - Poland will want to see the details of an EU proposal to allow Britain to suspend social payments to migrants before accepting a plan for persuading London to stay in the bloc, President Andrzej Duda said on Tuesday.

Poland, the biggest supplier of migrant labour to Britain, has been among the most vocal opponents of any measures it thought would discriminate its citizens working in Britain.

European Council President Donald Tusk's plan, which has to be accepted by all EU member states, seeks to address a British push for EU reform to stem migration and to boost British sovereignty.

While expressing some reluctance over the deal proposed by Tusk, a former Polish prime minister, including the so called "emergency brake" for migrants, Duda welcomed provisions that would bolster EU members' ability to stop legislation.

"The deal can be evaluated in different ways. I would like to underline one thing: it is a preliminary deal and it will be subject to proper negotiations," Duda told TVP Info news channel.

"Free movement of workers and services is a fundamental value of the European Union," he said. "There is a clause (in the deal) saying that in the case of a sudden influx of wage migrants some payments could be curbed. We will see what the interpretation (of the clause) is," he added.

EU leaders are due to discuss the proposal at a summit on Feb. 18-19, but talks will continue in the coming days. British Prime Minister David Cameron travels to Warsaw on Friday. His Polish counterpart, Beata Szydlo, will be in London on Thursday, although is not scheduled to meet Cameron then.

Szydlo's eurosceptic government, which was elected in October, has long called for Brussels to have less say over national issues. The ruling Law and Justice party (PiS) is also against Poland joining the euro zone any time soon.

"The deal includes important elements regarding strengthening sovereignty," Duda said.

(Reporting by Adrian Krajewski; Editing by Justyna Pawlak and Alison Williams)