United Kingdom opt-outs from EU legislation in the context of "Opt-outs in the European Union"

The 2015–2016 United Kingdom renegotiation of European Union membership was an unimplemented non-binding package of changes to the United Kingdom's terms of its European Union (EU) membership as a member state and changes to EU rules which were first proposed by Prime Minister David Cameron in January 2013, with negotiations beginning in the summer of 2015 following the outcome of the UK General Election. The package was agreed by the President of the European Council Donald Tusk, and approved by EU leaders of all 27 other countries at the European Council session in Brussels on 18–19 February 2016 between the United Kingdom and the rest of the European Union. The changes were intended to take effect following a vote for "Remain" in the UK's in-out referendum in June 2016, at which point suitable legislative proposals would be presented by the European Commission. Due to the outcome of the referendum in which the electorate voted by 51.9% to 48.1% to leave the bloc, the changes were never implemented and subsequently the United Kingdom withdrew from the European Union in January 2020.

The renegotiated terms were in addition to the United Kingdom's existing opt-outs in the European Union and the UK rebate. The changes were legally binding insomuch as the intentions and statements made by the EU leaders were enshrined in an international treaty. The implementation of some of the changes would have required legislation by the European Parliament or treaty change within the EU and so the details may have altered, although it would be hard for the European Commission or the European Parliament to directly defy national governments.