EU 'firm' on citizens' rights after Brexit: negotiator

European commission member Michel Barnier listens during a session on Brexit preparations on March 22, 2017 at the European Parliament in Brussels

EU Brexit negotiator Michel Barnier warned London Tuesday on the eve of its Brexit launch that the bloc would be "firm" on the rights of European citizens living in Britain.

"#Brexit made EU citizens worry about their future in EU27 and UK. EU will be firm on their rights @The3Million #citizensfirst," former French minister Barnier tweeted in English.

Brussels insists that Britain's divorce, including issues such as the rights of more than three million EU nationals in the UK, must be dealt with before any future trade relationship.

Last week Barnier, who was previously a European commissioner, said the amount that the EU says Britain must pay before it leaves, and the future of the border in Northern Ireland, must also be settled.

The European Union meanwhile tweeted a picture of Barnier's negotiating team around a table, with him sitting at the front.

While Barnier was warning Britain, European Council President Donald Tusk and European Commission chief Jean-Claude Juncker stayed silent on the issue during their public appearances on Tuesday.

Tusk is due to receive a letter from British Prime Minister Theresa May on Wednesday formally notifying the EU of its intention to leave, and launching the two-year negotiation process.