David Cameron faces a tough task to persuade the EU to make reforming treaty changes after Poland's ambassador to London warned there was "no appetite anywhere" for it.
However, the Polish position appeared somewhat at odds with the view of German Chancellor Angela Merkel, who said Berlin would be open to treaty change of it kept Britain in the EU.
Speaking to the Evening Standard, the ambassador, Witold Sobków said: "There is no appetite for treaty change anywhere. It's not just Poland."
The UK Prime Minister will need to support of countries such as Poland because German and France are understood to have struck a secret deal to oppose treaty change for greater integration.
The Tory government hopes to introduce reforms that would restrict benefits for migrant workers from EU member states. That be against treaty agreements concerning the free movement of people and labour.
Sobków said that this was as "red line" for Warsaw, along with another on welfare restrictions not "discriminating" against Polish citizens, which Cameron proposed in talks with Poland's Prime Minister, Ewa Kopacz, last week.
No treaty changes and free movement of people is sacrosanct
Poland is keen to help the UK to find "solutions" to keep it in the EU, possibly through reforms using secondary legislation rather than treat change. Warsaw would also help change the wording of some agreements so that Britain is freed from "ever closer union", protecting EU countries outside the eurozone and powers for national parliaments.
Sobków said it "would be helpful to get a list of concrete proposals, very precise proposals, and then we could sit and talk about it. In every negotiation, we can find some compromises.
"But for us no treaty changes and free movement of people is sacrosanct."
Meanwhile, speaking to the BBC, Merkel said she was confident that conditions could be created for the UK to stay in and she is "not losing sleep" over it.
"I'm optimistic that if we all want it, we'll find a good solution," she said.
"It's not about losing sleep over this, but about doing our work and creating the necessary preconditions for Britain to remain in the EU."
She added: "Some of the things that David Cameron is asking for I can support. There are other points where we have a different opinion, but we have always been able also to pursue a Europe at different speeds, to find opt-out solutions for example."
When asked whether EU treaties would be changed, she replied: "If that is really necessary then we have to consider it."