@Walter Hinteler,
The UK will leave the EU on 29 March - at least that is the current situation. But a postponement of the Brexit is still under discussion in order to avoid chaos. However, this puts the EU in a difficult position, because: if the British have not resigned in May, they would have to take part in the European elections. British politicians may be given such a mandate, even though the country is leaving the EU a few weeks to months later. But excluding a country from the election does not work either.
A so-called "Room Document", seen by "Spiegel", shows the concrete plan of the EU. According to this, the United Kingdom must leave the EU or participate in the elections by 1 July at the latest.
As the Parliament will meet for the first time on 2 July, "no extension beyond 1 July should be granted if no European elections have taken place on the set date", the paper quotes from the document. If the United Kingdom does not participate in the election, "the extension should end before the European Parliament meets on 2 July".
Report (in German) @ spiegel-online