May's unexpected meeting with European Commission President Jean-Claude Juncker and chief negotiator Michel Barnier follows Barnier's warning last week that the latest round of talks ended in a "disturbing deadlock" over Britain's financial obligations to the bloc.
EU estimates suggest Britain must pay from 60 billion euros to 100 billion euros (USD 80 billion to USD 120 billion) to settle commitments it has made while part of the EU, including development projects and the pensions of civil servants. Britain has rejected such figures.
British Foreign Secretary Boris Johnson on Monday urged the EU to speed up talks and start a discussion of the future relationship with the UK, which is set to leave the bloc in March 2019.
It is time for "the great ship to go down the slipway and onto the open sea and for us to start some serious conversations about the future and the new relationship," Johnson told reporters.