BRUSSELS- The European Union has yet to set a date for a summit to respond to Britain's notice of withdrawal but it should be between four and six weeks after March 29, the day when Britain will trigger Brexit, an EU source said on Monday.Speaking after London announced that Prime Minister Theresa May would start the two-year withdrawal process by writing to EU summit chair Donald Tusk next Wednesday, the source said this did not leave enough time to convene the other 27 leaders on April 6-7, dates that had been penciled in for a meeting.EU officials have previously said that a notification just before May's self-imposed end-March deadline could mean a summit in early May was the most likely timing.Preparations may be slowed by holidays - around Easter on April 16 and on May 1. Brussels also wants to avoid clashing with the two-round French presidential election on April 23 and May 7. Officials say they would prefer to hold the summit before President Francois Hollande steps down around mid-May.Tusk reiterated on Monday that he would send governments a draft of Brexit negotiating guidelines within 48 hours of May's letter, which will set out Britain's demands for the talks. The draft is broadly ready, based on what May said in a key speech in January, but may need to be fine-tuned, EU officials say.The summit will endorse the final guidelines and mandate the EU's negotiator, Michel Barnier from the European Commission. He will then the send the Council his recommendations for how to run the negotiations. A Commission spokesman said on Monday Barnier would do this "immediately" after the summit.Before negotiations with Britain can actually start, the Council, in the shape of ministers sitting in the General Affairs Council (GAC), must approve the recommendations by issuing legally binding "negotiating directives" to Barnier.The European affairs ministers who sit on the GAC are due to meet in Brussels on May 16, although the body, currently chaired by the Maltese government, could convene at another date.