Europe’s political leadership is braced for difficult news on Sunday night, when the voting results will start to come in. The results will trigger contentious negotiations, first about the composition of the European Parliament and then about the composition of the European Commission.

Exit polls will give an early indication, albeit not wholly reliable, of the shape of the new Parliament, but they will be published in only 15 of the 28 member states. Although results cannot be published officially until after the last polling station closes in Italy on Sunday at 11pm, exit polls will come out in the Netherlands and the UK, on Thursday night.

Exit polls are expected in Germany, the EU’s biggest member state, by early evening on Sunday and at about 9pm-10pm in Poland and Spain. The results will come in from the 28 member states during Sunday night but will probably not be complete until Tuesday. But by Monday morning, when results from France are expected, the picture will be almost complete. National electoral laws set different counting procedures and different deadlines for submission of the results.

The leaders of the Parliament’s political groups are to meet in Brussels on the morning of Tuesday 27 May. Herman Van Rompuy, the president of the European Council, has convened an informal meeting of the national leaders in Brussels that afternoon.

The first task of that meeting will be to assess the composition of the new Parliament and the relative strengths of the various political groups. Van Rompuy will be looking to set out the procedures for choosing the next president of the European Commission.

The Parliament will be asserting its role in the selection and, depending on the results of the elections, some MEPs may urge Van Rompuy to choose the candidate of the group that won most seats in the Parliament. But at that stage, it is possible that there will not be enough results in to determine which political party can command a majority in the Parliament.

MEPs will come to Brussels in the first week of June for political group meetings. Negotiations on the formation of political groups – particularly between Eurosceptic parties – will be particularly significant. Those meetings are to continue throughout June.

The meeting of the G7 in Brussels on 4-5 June will present another opportunity for informal discussions between Angela Merkel, Francois Hollande, Matteo Renzi and David Cameron.

On 26 June, all 28 national leaders will meet in Brussels for a pivotal European Council that is supposed to settle on a nominee for European Commission president.

On 1 July, the new European Parliament will begin its first plenary session in Strasbourg. It will elect the European Parliament president and vote on confirming the decisions that have been reached in June over committee appointments.

MEPs will also that week begin discussions about the Council’s nominee for Commission president. However, it is the second plenary session, to begin on 14 July, that will be dedicated to considering this appointment. The Parliament can either vote to confirm the nomination, or delay the vote until the autumn if there is no majority for the nominee.