It comes after what both sides said was a "robust but constructive" meeting between Mrs May and European Commission President Jean-Claude Juncker.
Mrs May wants to prevent the UK being "trapped" in the backstop - the plan to keep the Irish border open.
Mr Juncker ruled out legally-binding changes to the backstop clause in the withdrawal agreement.
But he said the EU would be open to adding words to the non-binding future relations document that goes with the withdrawal agreement, following their meeting.
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