Londoners who want to live and work in Europe should be able to apply for "associate citizenship" of the EU, Sadiq Khan said.
Associate citizenship would let Britons retain rights on working and living in Europe and move freely between the UK and different EU states. It could mean that Britons who paid to apply for associated citizenship received a new passport or a stamp in their existing document.
Khan told reporters that he believed the idea had merit. "There's an opportunity for us to move forward with this and I've been pleased with the response and it gives hope to London, as well as across our country", he said, following meetings with EU officials.
There's an opportunity for us to move forward with this and I've been pleased with the response and it gives hope to London, as well as across our country"
London's mayor travelled to the Belgian capital to meet the EU's chief Brexit negotiator Michel Barnier and European Parliament president David Sassoli.
With the backing of the former prime minister of Belgium Guy Verhofstadt, Khan called for the idea to be at the "the heart" of the negotiations over the future relationship.
Khan said that rejoining the EU was not foreseeable in the "short to medium term" but that associate citizenship of the bloc could be one way to "make the best of Brexit".
The plans are unlikely to become reality though as it would require treaty change from the EU and the UK government has repeatedly said that freedom of movement will stop at the end of the year.
The idea had been first raised in late 2016 by Verhofstadt, who was then the European parliament's Brexit coordinator.
The mayor wants it to be included in negotiations on the terms of the UK's departure. However, that is likely to conflict with EU law and has already been rejected in court cases brought by Britons in EU states.
More than one million Europeans live in London and EU citizens are able to vote in the forthcoming mayoral elections.