European Union leaders have been told to be ready for a Brexit summit at the end of the first week of April, EU sources said on Wednesday, although the final date will depend on British Prime Minister Theresa May.

A summit of the other 27 national leaders to respond to May's formal notification of Britain's departure appears in an internal EU calendar seen by Reuters on Thursday, April 6, "to be confirmed".

An EU source said summit chair Donald Tusk had asked last month that leaders in the European Council keep that and the following day free for a summit which will set out guidelines for EU negotiators, based on what deal May asks for.

However, the EU will need about four weeks after her letter triggering withdrawal under Article 50 of the EU treaty to be ready for the summit. The April 6-7 dates were pencilled in early February in the expectation May might write in mid-March.

But opposition in the British parliament to launching the process, including a government defeat late on Wednesday, may have put that date in jeopardy. The government had hoped to be free to launch the Brexit process from March 7 but the parliamentary process cannot now end before March 13.

That rules out any prospect that May might deliver the letter when she attends a Brussels summit on Thursday, March 9.

Some diplomats believe May might now choose to wait until after Scotland's ruling nationalists finish a party conference on March 18, depriving their leader Nicola Sturgeon of a chance to press a case for secession on the basis of Scottish rejection of the kind of "hard Brexit" many expect to see in May's letter.

British ministers have said they would like, however, to avoid sending the divorce letter close to March 25, when the other 27 leaders will meet in Rome to celebrate 60 years when the bloc's founding treaty was signed there in 1957.

For the EU, Easter holidays and France's two-round presidential election on April 23 and May 7 will complicate any summit to be held later than April 7. EU officials say a summit would be possible the following week, before Easter, April 16.

If May only writes in late March, a summit is more likely to be delayed until the last week of April, EU officials have said.

Visiting Brussels on Wednesday, British Northern Ireland Secretary James Brokenshire told Reuters the government could not be specific on the date but was confident of triggering Article 50 by the end of the month, as May has long promised.