Londoners who want to live and work in Europe should be able to apply for "associate citizenship" of the EU, Sadiq Khan said today.
The Mayor, who proposed the idea in a letter to Prime Minister Boris Johnson yesterday, was in Brussels today to meet leading European politicians in a bid to retain links between the capital and the EU.
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.
The idea has been championed by MEP Guy Verhofstadt, until recently chairman of the European Parliament's Brexit steering group, who Mr Khan was meeting today.
The Mayor wants it to be included in negotiations on the terms of the UK's departure from the EU.
However, the idea 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.
Mr Khan, who opposed Brexit, said he was deeply concerned that millions of pro-EU Londoners had lost their rights.
He said it could form an “olive branch” to pro-EU Britons “heartbroken” by Brexit.
He told the Standard: "It's a great way to unite the country. It's an olive branch. We have left the EU but for those of us Remainers who want to stay close to the EU, this is one way of doing that.
"It may need a treaty change. It may need a new treaty. If under the current rules it can't be done, there is no reason why the EU, if they were amenable to this, couldn't explore this further."
He added: “Like so many Londoners, I am heartbroken that we are no longer a member of the European Union, but that doesn’t mean our country’s future can’t be closely linked with the rest of Europe.
“The Prime Minister says his job is to bring the country together and move us forward and I cannot think of a better way of reconciling the differences between British voters who wanted to leave, and the millions of Londoners and British nationals who still feel and want to be European.
“There would be support from millions of Londoners and British nationals who are devastated they are losing their rights as EU Citizens. As the UK and EU start their next phase of negotiations, I want this issue of associate citizenship to be at the heart of talks about our future relationship.”
Siobhan Benita, his Lib-Dem mayoral rival, who proposed a "London visa" when she visited Brussels last year, said it was "too little, too late" from Mr Khan.
She told the Standard: "Londoners would still be in the EU if his Labour party had supported the LibDems in their unequivocal opposition to Brexit."
Ursula von der Leyen, president of the European Commission, has said the EU is open to exploring the idea as part of wider negotiations with the UK.
Mr Khan is also due to meet Michel Barnier, the EU’s chief Brexit negotiator, David Sassoli MEP, President of the European Parliament, and Frans Timmermans, vice-president of the European Commission.
Mr Verhofstadt said: “The Maastricht Treaty created the concept of 'European Citizenship' and I am in favour of using this now as a basis for people who want to keep their link with Europe.
"It is the first time in the history of our union that a member state leaves, but it is not because the UK government wanted exit that individual citizens have to lose their connection with the continent.”
Independent mayoral candidate Rory Stewart said: “This is only a sound bite - the policy cannot work. The Mayor has a responsibility to produce a much more detailed position on Brexit - that protects London. I have such a plan.
"And if I were Mayor I would be focused on making detailed arguments about a future immigration system - as well as service agreements - to keep London as close as possible to Europe."
Tory candidate Shaun Bailey said: “Three more Londoners were stabbed on the weekend and a teenager is fighting for his life as we speak, and where is our Mayor? In Brussels, announcing a gimmick he has no powers to implement.
“The Mayor has got to focus on his actual responsibilities; he should be in London, urgently speaking with the Met Police, over whom he does have actual powers, about how to stop kids being stabbed to death on our streets. We need a change in leadership, we need a new Mayor whose priority is keeping Londoners safe.”