This timeline shows the key events in the negotiation process behind the new settlement for the UK within the EU. The new settlement was subject to a referendum in Britain on 23 June 2016.
The outcome of the referendum will be discussed by the European Council on 28-29 June 2016.
"In a free and democratic process, the British people have expressed their wish to leave the European Union. We regret this decision but respect it," said President Tusk, President Juncker, President Schulz and Netherlands Prime Minister Rutte in their joint statement.
"As agreed, the 'New Settlement for the United Kingdom within the European Union', reached at the European Council on 18-19 February 2016, will now not take effect and ceases to exist. There will be no renegotiation," they added.
"We are prepared for this negative scenario. There will be no legal vacuum," said President Donald Tusk following the official results of the UK referendum.
The new agreement was subject to a referendum in Britain on 23 June 2016.
Following intense negotiations, EU leaders achieved a deal which strengthens Britain's special status in the EU. It is a legally binding and irreversible decision by all 28 leaders.
Following the publication of President Tusk's proposal, discussions on the deal continued at all levels. President Tusk met again with Prime Minister Cameron in the margins of a conference on Syria, in London on 4 February and they spoke on the phone on 15 February.
Meetings also took place between Tusk's team and the envoys (sherpas) and permanent representatives of all 28 member states on 5 and 11 February. They discussed the details of the proposal in order to seek to reach an agreement among all member states at the February European Council.
A meeting was held on 8 February between President Tusk´s negotiating team and the EP's representatives, Guy Verhofstadt, Elmar Brok and Roberto Gualtieri. President Donald Tusk also spoke with President Schulz on 10 February and met the three representatives on 12 February.
As of 12 February, President Tusk has also been meeting several other EU leaders: the Prime Minister of Belgium, President Hollande, Chancellor Merkel, the Prime Minister of Greece, the President of Romania and the Czech Prime Minister, who holds the presidency of the Visegrad Group (comprised of the Czech Republic, Hungary, Poland and Slovakia).
The aim of his consultations was to secure a broad political support for his proposal and find solutions to all unsolved issues.
On 2 February 2016, President Donald Tusk put forward a proposal for a new settlement addressing all the concerns raised by Prime Minister Cameron.
This was the basis to negotiate a compromise with all 28 member states and to reach an agreement at the European Council on 18-19 February.
The December European Council debated the UK plans for the in/out referendum and agreed to find solutions in all the four areas of concern (economic governance, competitiveness, sovereignty, social benefits and the free movement of persons) at their February meeting.
In his remarks following the debate on the UK issue, President Tusk announced that he will keep on working closely with Prime Minister Cameron and the European Commission to draft a proposal in the run-up to the February European Council.
On 7 December, President Donald Tusk addressed a letter to the European Council on the issue of the UK in/out referendum and the concerns of the British people over UK membership of the European Union.
"All in all it is my assessment that so far we have made good progress. We need some more time to sort out the precise drafting on all of these issues, including the exact legal form the final deal will take. We also have to overcome the substantial political differences that we still have on the issue of social benefits and free movement.", wrote President Donald Tusk.
He also announced a timeline for decision: "The December European Council should address all the political dilemmas related to this process. Based on a substantive political discussion we should be able to prepare a concrete proposal to be finally adopted in February."
On 10 November 2015, in a letter to President Donald Tusk, UK Prime Minister, David Cameron, set out the four areas where he is seeking reforms.
The European Council was informed about the process ahead concerning the UK plans for an (in/out) referendum. Prime Minister Cameron indicated he will set-out the UK's specific concerns in writing by early November.
The European Council decided to revert to the matter in December.
Following the informal meeting of EU leaders on 23 September 2015 on migration and refugees, President Tusk and Prime Minister Cameron had a bilateral meeting to discuss the state of play on the UK in/out referendum.
The UK Prime Minister, David Cameron, set out his plans for an (in/out) referendum in the UK. The European Council agreed to revert to the matter in December.
Ahead of the meeting, President Tusk also had the opportunity to hold a bilateral meeting with Prime Minister Cameron.