Mark Carney received a rapturous reception in the European parliament when he gave his speech on Thursday welcoming a proposal for Canada to become the EU’s first-ever associate member.

Politicians queued to shake his hand and take selfies. One described the speech as a historic moment. “Europe has extended a hand, and Canada has taken it,” said the German Green MEP Terry Reintke, who borrowed a Canadian ice hockey shirt for the occasion.

One day earlier, in the same debating chamber, the European Commission president, Ursula von der Leyen, issued the unprecedented invitation to Canada – an idea quickly derided by Donald Trump, who threatened Europe with “heavy tariffs” if it became a reality.

But the prospect of Canada’s associate membership is uncertain, not least because no one is sure what it means. “I’m not quite sure why she [von der Leyen] used that term because there’s simply no legal definition,” said one senior politician. “This has just raised lots of questions. So people are talking more about what an associate membership means instead of … this unique partnership and alliance for the future that EU-Canada relations should be about.”

The German chancellor Friedrich Merz’s spokesperson welcomed the initiative for a close partnership with Canada and said Germany was “open to discussing new models that reflect the special significance of the relationship”, but called for the associate membership label to be “reconsidered”.

Merz himself has floated “associate membership” for Ukraine, as a stepping stone to joining the bloc, but this was quickly dismissed by other member states.

One EU diplomat said von der Leyen “seemed a bit far out in floating this” and that even before Wednesday’s speech member states had worried that she was overpromising on Canada. A second diplomat said it was “an empty phrase” and it would be up to member states to define the substance.

Any new trading and cooperation relationship with a foreign country would need to be approved by the EU’s 27 member states.

There is a lot of common ground: both sides want to deepen cooperation on AI, energy security, critical raw materials, quantum computing and work together in the Arctic – Carney noted that Canada shared an Arctic border with Denmark, referring to the maritime border with Greenland.

But difficulties went unspoken. Carney described the 2016 EU-Canada comprehensive economic and trade agreement as “one of the world’s most successful trade agreements”, but 10 EU member states have yet to ratify it, including France, Italy and Poland. With all three countries heading into an election year, that looks unlikely to change.

While both sides extol the virtues of deeper trade between allies, they are also pursuing policies that appear to contradict that goal. Last year Canada introduced a buy Canadian policy, to ensure Canadian suppliers get priority in public contracts. The EU is debating its own buy European law, known formally as the Industrial Accelerator Act. The IAA, now being debated in the EU, would oblige member states to meet “made in Europe” content requirements when spending public money on green technologies and electric cars.

Associate membership also raises questions about integration into the EU’s tightly guarded single market. Carney proposed “seamless digital trade in non-agricultural goods and a wide range of services”. But the EU has long insisted that the four freedoms of its internal market – free movement of goods, services, labour and capital – are one package that cannot be split and require alignment with EU rules.

Now the applause has died down, there are plenty of tricky issues to work out, even for the closest of allies.