Beata Szydło says she did not ‘see eye to eye’ with British PM over issue following late-night talks in Warsaw

David Cameron has been warned that he faces two months of tough negotiations on his EU reform proposals after the Polish prime minister said Warsaw did not “see eye to eye” with the UK over plans to restrict access to in-work benefits.

In a sign of the deep unease in eastern Europe at the proposed restrictions, Beata Szydło warned Cameron that the “basic principles” of the EU’s rules on freedom of movement must be respected.

The prime minister has spent the last two days visiting Bucharest and Romania before an EU summit in Brussels next week where he had hoped to reach a deal.

But Cameron has agreed to delay the final round of negotiations until the next EU summit in February after Donald Tusk, the president of the European council, said EU leaders had failed to agree to the plan to ban EU migrants from claiming in-work benefits for four years.

Poland and other eastern European countries, whose citizens would be hit harder than citizens from wealthier member states, believe the ban breaks the EU’s laws on discrimination.

The Polish prime minister warned Cameron not to infringe the EU’s rules on free movement and said she had raised concerns about the UK’s proposals on “the welfare system and child benefits”.

In addition to the proposed ban on in-work benefits, the UK is attempting to ban EU migrants working in the UK from sending back child benefit for children at home.

The ban on in-work benefits would require a treaty change because EU citizens are entitled to be treated in the same way as UK citizens in the workplace, including topups to their wages through tax credits. Szydło, who became prime minister last month, said: “There are also discussions and issues about which we do not see eye to eye today.”

But Cameron took heart when Szydło signalled Poland’s determination to keep its historic wartime ally in the EU. Szydło spoke of a “common direction” and said was hopeful that a solution could be found to keep the UK in Europe. “I believe that these issues will be further discussed by us. We will be talking about them, we will try to solve them together in consensus.”

The Polish prime minister signalled a degree of support for Cameron’s plans to make it all but impossible for EU migrants to claim out-of-work benefits. Szydło said: “We fully accept the right of the United Kingdom to take sovereign decisions with regard to welfare policy. We want to find a solution that is acceptable to the United Kingdom.”

Poland has focused its concerns on the restrictions on in-work benefits, rather than on out-of-work benefits, because the vast majority of its citizens that travel to the UK do so to work. Britain alone has 850,000 registered Polish nationals as residents, though the real figure is estimated to be closer to 1.3 million. Each year 20,000 children are born in Britain to parents with Polish nationality.

Cameron moved to reassure his Polish counterpart that he was not seeking to discriminate against Poles in Britain and stressed that he was not seeking to undermine the EU’s founding commitment to free movement.

He said: “Even on the most difficult issue of welfare, we have agreed to work together to find a solution. I support the principle of free movement and I greatly value the contribution that many Poles and other Europeans make to Britain.”

But the prime minister added that the UK needed to impose some restrictions on benefits to reduce migration. EU migrants will effectively be banned from claiming out-of-work benefits under welfare reforms.

Cameron said: “The challenge is the scale of the vast movement of people we have seen across Europe over the last decade and the pressure that can put on public services. That is the problem we need to address and I believe with the type of political will I have seen here in Poland we can find a way.”

The prime minister ended his visit to the Polish capital with an impassioned call for Britain to remain in the EU. “I think the right outcome is to be part of a reformed EU. European leaders believe the EU benefits from having the UK inside,” he said at a press conference with Szydło.

Cameron had earlier warned that the “migration crisis” might prompt people to support an exit from the EU. He told the Spectator: “I think with both the eurozone crisis and the migration crisis, the short-term impact is for people to think: ‘Oh Christ, push Europe away from me, it’s bringing me problems.’”

But the prime minister said he hoped that on reflection people might decide to remain in the EU because it is “on our doorstep”.

He added: “I think the longer-term reaction might actually be, well if they are going to have a single currency and they are on our doorstep and they are going to try and make it work, let’s make sure our relationship with them works and then we have safeguards, not least for our vital financial services industry so that the system doesn’t work against us.”

Gaining Poland’s support is key to Cameron’s strategy, not only because Poles make up the single largest group of immigrants to Britain but because of the country’s regional importance. Poland is the biggest economy in the group of so-called Visegrad countries – the Czech Republic, Hungary and Slovakia. Poland is influencial from north to south in the group of countries often referred to as “New Europe”, from Estonia to Croatia.

Cameron alligned the Conservatives with the populist Law & Justice party, which won Poland’s parliamentary elections on 25 October. “We are natural partners. It is great to see a sister party back in power,” he said.

Law & Justice is part of the same group as the Tories in the European parliament. Several of its moves in its first few weeks in power have raised concern at EU level. Decisions so far include the removal of the provision of IVF from the public health system, the sacking of the head of the intelligence service and controversial new appointments to Poland’s constitutional tribunal.