DAVID Cameron’s continental charm offensive to sell his EU reform package has received a boost after he won the backing of a key Polish party leader while Denmark’s premier made clear that he supported “all the elements” of the proposed welfare curbs.

But the Prime Minister was also warned by Martin Schulz, the President of the European Parliament, that several EU leaders found Britain “demanding,” noting: “Many of my colleagues say behind closed doors: 'Don't stop a rolling stone. If the Brits want to leave, let them leave.'"

It may be no coincidence that next week, ahead of his keynote speech in Hamburg on Friday, Mr Cameron will be in Brussels to meet the leaders of the different parliamentary groups to persuade them to back his reform package.

Poland, which has an estimated 800,000 residents in the UK, has been widely seen as the most resistant to the PM’s plan to restrict in-work benefits to EU migrants as it feared it would hit its citizens disproportionately hard.

But on a visit to Warsaw, Mr Cameron received support from Jaroslaw Kaczynski, a key Polish power-broker, who leads the governing Law and Justice Party.

After a meeting, Mr Kaczynski said he was "satisfied" with the proposed benefit curbs, saying: “We have fully secured the rights of Poles living already in the UK.”

While No 10 has refused to comment on the details of the so-called emergency brake, it is thought the restrictions could only apply to new and not existing migrants, who live and work in Britain.

And in a potential sign of a quid pro quo, the PM made a strong reference to the UK’s “strategic partnership” with Poland with a focus on securing Nato’s eastern flank against Russia.

Diplomatic sources said that Poland would support the emergency brake since any decision to use it would have to be passed by MEPs under the supervision of the European Court of Justice.

In Denmark, the PM won the fulsome support of his counterpart Lars Lokke Rasmussen, who hailed the UK as a voice for "common sense”.

Mr Rasmussen said he backed "all the elements" on access to welfare benefits, saying the emergency brake was "perfectly understandable and acceptable" to Denmark.

He added: "Denmark will work hard to ensure a result, which can help the British Government win a referendum and remain a key member of the European Union."

Mr Cameron told reporters “good work” had been done but there were still two weeks to go until the crunch EU summit on February 18/19.

“I still need all 28 countries to agree to the changes, there are still important details to be filled in and work to be done on this negotiation,” admitted the PM.

"But if you look at what we are actually achieving, in the areas of things people care about, that is what we promised in our manifesto and that is what we are delivering," he added.

Meantime, tensions within the Brexit camp have boiled over again with leading Labour and Green Party figures withdrawing support for Vote Leave.

Labour MP Kate Hoey said she no longer endorsed the group's bid to be officially designated as the Out campaign for the EU referendum while Green peer Baroness Jones said she was standing back from the organisation because it lacked "judgement".

The developments prompted Ukip leader Nigel Farage to warn that the referendum would not be won without "left-wing support".