European Council president Donald Tusk has told British MPs if they want to negotiate British citizens' rights in the EU, they need to trigger Article 50 now.
In a response to complaints from MPs on monday, Tusk said to secure the fate of EU citizens in the UK and UK citizens living and working in Europe, negotiations need to "effectively begin in December" .
The letter came in response to a letter signed by 80 MPs, which called for an early agreement on the rights of British citizens living in EU countries and EU citizens living in the UK, in order to end "anxiety and uncertainty".
The letter, which was organised by MPs Michael Tomlinson and Steve Baker, complained that the European Commission was refusing to discuss the subject until Article 50 was formally triggered - scheduled for March.
The MPs, including Michael Gove, John Whittingdale and Iain Duncan Smith, said they were "extremely concerned" about Europe's "worrying [indifference]" to the fate of British citizens living and working in the EU.
Today Tusk said MPs' concern for the status of citizens both in the UK and Europe "bodes well for future negotiations", but added that the main source of "anxiety and uncertainty" was Brexit itself.
"Would you not agree that... the only way to dispel the fears and doubts of all the citizens concerned is the quickest possible start of the negotiations on Article 50 of the Treaty?"
Earlier today Eurogroup president Jeroen Dijsselbloem said he did not expect London to remain as Europe's financial hub after Brexit, adding the Union will have to take a "firm stand" against it.
"We cannot allow a third country to have access, full passporting rights, to financial service markets in Europe if at the same time we allow them to deviate in terms of capital standards, requirements, consumer protection, etc etc," Dijsselbloem said.
Meanwhile, Downing Street officials were put on the defensive yesterday after a photographer tweeted a picture of an aide carrying a notebook which appeared to include scribbled notes on the UK's Brexit negotiation tactics.
"What's the model? Have cake and eat it," the notepad read - while it pointed to a "very French negotiating team".