May's spokesman insisted the prime minister has an "absolute determination" to make sure Britain leaves the EU with a deal as soon as possible.
The EU's chief Brexit negotiator Michel Barnier said at a press conference in Brussels Tuesday that May that should not take it for granted that the EU will agree to an extension of Britain's departure date, scheduled in just 10 days.
At the press conference in Brussels, Barnier said delaying Brexit could bring economic and political costs, and the EU would want reasons for an extension.
Any extension will need the support of the other 27 EU members.
May will meet the leaders of the other 27 EU members at an EU summit taking place on Thursday.
May is due to take questions Wednesday at her usually PMQ's, Prime Minister's Questions, in the House of Commons when she could be asked to comment on the latest EU situation.
The next major Brexit debate is scheduled to take place in the House of Commons on Monday when MPs will be told the result of May's talks.
Under current legislation Britain's membership of the EU will come to an end next week, on May 29, unless a change is made to the law that set the departure date.