WARSAW/COPENHAGEN (Reuters) - Prime Minister David Cameron secured the support of Poland’s most powerful man on Friday for a proposed deal to keep Britain in the European Union, though an opinion poll suggested voters may still reject continued membership in a planned referendum.

To win a vote expected in June on staying in the EU, Cameron says he needs a pact to curb benefits for new migrant workers from EU countries, an opt-out from moves towards political union, more powers for national parliaments and safeguards to ensure Britain is not put at a disadvantage by being one of nine EU members outside the euro currency zone.

He achieved an important step towards that goal when Jaroslaw Kaczynski, leader of Poland’s ruling Law and Justice party and the country’s top decision maker, gave his blessing to the proposed arrangements for new migrant workers.

Negotiators from all 28 EU countries meanwhile held a first discussion in Brussels of the package put forward by European Council President Donald Tusk. Diplomats said the day-long meeting was broadly supportive and threw up no roadblocks.

The so-called sherpas will meet again on Feb. 11 with the aim of reaching a deal at a Feb. 18-19 EU summit.

“There is a lot of positive energy and dedication from all sides to arrive at an agreement. That also goes for the Brits who have been very professional about how they handle these negotiations,” one participant said.

Another said there were still differences on details of the proposals on welfare rights and on the relationship between euro zone and non-euro countries, but there was a strong desire to find a quick solution to keep Britain in the bloc.

But a third diplomat said Kaczynski’s timely endorsement of the most controversial item in the package had disarmed any strong opposition around the EU table.

“Everything seemed very well pre-cooked... What kind of fierce debate do you expect re free movement if during the meeting Cameron successfully courted Kaczynski?” the envoy said.

Proposals to allow British authorities to withhold in-work benefits for up to four years from EU citizens moving to work in Britain are under intense scrutiny, especially from Poland, the biggest source of Britain’s migrant labour force.

After meeting with Cameron in Warsaw, Kaczynski said he was satisfied because the rights of some 600,000 Poles already working in the UK would be fully preserved.

“We have gained really very, very much,” said Kaczynski, who is also a former prime minister and the twin brother of late president Lech Kaczynski.

“Poland has ... gained here really very much, full safety, above all, for all those who are in Britain right now, but also that those who have children in Poland will continue to receive benefits, they may be adjusted, but they will get them anyway.”

Prime Minister Beata Szydlo said she fully endorsed Cameron’s proposals on improving competitiveness, removing red tape and granting proper significance to national parliaments but wanted to discuss the question of welfare benefits.

One detail yet to be finalised is whether curbs on child benefit for migrants whose children stay in their home country will apply only to newcomers or also to existing EU workers in Britain, diplomats said.

Cameron said later during a visit to Denmark that any agreement with the EU would be irreversible.

“There’s no way we’re going to agree to reverse it,” Cameron said at a news conference with Danish Prime Minister Lars Lokke Rasmussen, who said the proposed deal was “a solid answer”.

A Union Jack flag flutters next to European Union flags ahead of a visit from Britain's Prime Minister David Cameron at the EU Commission headquarters in Brussels, Belgium, January 29, 2016. REUTERS/Francois Lenoir

TROUBLE AT HOME

Cameron’s sought-after accord may be taking shape in Brussels but he still faces a deeply sceptical electorate. A new poll showed the campaign for Britain to leave the EU has taken a nine-point lead over the rival “remain” campaign.

Nineteen percent said they did not know or would not vote.

British polls, however, have faced questions about their methodology since they failed to predict Cameron’s victory in the 2015 national election, and they have varied widely on the EU issue in recent months. A ComRes poll last week showed the “in” campaign held an 18-point lead over those who wanted out.

A British exit would shake the European Union to its core, ripping away its second largest economy and one of its top two military powers. Britain’s pound sterling could weaken as much as 15-20 percent against other major currencies if there is a vote to leave, according to Goldman Sachs and Citi.

Pro-Europeans say an exit would damage Britain’s economy by excluding it from the vast EU single market, and could trigger the break-up of the United Kingdom by prompting another independence vote by pro-EU Scotland. Opponents of EU membership say Britain would prosper outside.

The survey for The Times newspaper, taken in the two days after Cameron set out a proposed deal, showed the biggest lead for the “out” campaign since the referendum wording was agreed last September.

The poll showed 45 percent of Britons would vote to leave the EU against 36 percent who want to remain, well up on the four-point edge the “leave” campaign held last week.

Eurosceptic members of his Conservative Party said the proposals in the outline deal were far too weak and the British press dubbed them a “farce”, a “joke” or a “delusion”.

“The negative press has pushed ‘Leave’ significantly ahead,” YouGov said. “It’s too early to say if the lead will persist or subside after David Cameron’s crunch talks in Brussels.”

German Chancellor Angela Merkel has repeatedly said she wants Britain to remain a member but has cautioned that Cameron must not overplay his hand.

Tusk’s plan to keep Britain in the EU goes “right up to the pain threshold” of what is acceptable in Germany, said Gunther Krichbaum, a member of Merkel’s conservative party and chairman of the European affairs committee in parliament.