David Cameron's push to rebrand the EU as a "multicurrency union" has triggered high-level concerns at the European Central Bank, which fears it could give countries such as Poland an excuse to stay out of the euro.
The UK prime minister wants to rewrite the EU treaty to clarify that some countries will never join the single currency, in an attempt to ensure they do not face discrimination by countries inside the eurozone.
Mario Draghi, president of the ECB, is worried the move could weaken the commitment of some countries to join the euro.
Beata Szydlo, the new Polish premier, has previously described the euro as a "bad idea" that would make Poland "a second Greece".
Mr Draghi shares concerns in Brussels that the EU single market could be permanently divided across two regulatory spheres, with eurozone countries facing unfair competition if there were a lighter-touch regime on the outside.
The idea of rebranding the EU as a "multicurrency union" was raised during a recent meeting in London between George Osborne, the UK chancellor, and Mr Draghi. Mr Osborne said last month that Britain wanted the treaty to recognise "that the EU has more than one currency".