The EU Commission President, Jean-Claude Juncker, has urged Belgium to grant citizenship to British EU staff worried about their post-Brexit status.
About 1,100 UK citizens work for the EU in Brussels and Luxembourg.
Mr Juncker called Belgium a kind host and asked its prime minister, Charles Michel, to "show the same generosity when it comes to granting Belgian citizenship" to British EU staff.
When the UK leaves the EU next March Britons will lose their EU citizenship.
The UK and EU have already pledged to protect citizens rights after Brexit, but that does not mean granting nationality.
Article 49 of the EU staff rules states that "an official may be required to resign" if he or she loses their EU citizenship and is no longer a national of an EU member state.
Many British EU staff are longstanding residents with families.
An internal EU Commission document quoted by Politico news last month said Article 49 would not mean British staff losing their EU jobs, apart from cases involving "conflicts of interest or international obligations".
Responding to Mr Junckers plea during a European Parliament debate, Mr Michel said Belgiums citizenship law in the context of Brexit was "contradictory", but he did not specify the difficulties.
"The government is examining the judicial possibilities on this question, which affects a number of people who have been living in our country for a long time," he said.
European Commission spokesman Margaritis Schinas said later that Mr Junckers remarks were meant as a reminder for the Belgian leader. "We live up to the promise that our colleagues of British nationality should be given maximum guarantees to stay, not only with their employment but also if they want to stay as Belgian citizens. But this is in entirely in the hands of the Belgian government, not ours," he said.