When they attended the May 2012 NATO summit meeting , UK Prime Minister David Cameron , Foreign Secretary William Hague and Ed Llewellyn discussed the idea of using a European Union referendum as a concession to energise the Eurosceptic wing of the Conservative Party . Cameron promised in January 2013 that , should the Conservatives win a parliamentary majority at the 2015 general election , the British government would negotiate more favourable arrangements for continuing British membership of the EU , before holding a referendum on whether the UK should remain in or leave the EU . The Conservative Party published a draft EU Referendum Bill in May 2013 , and outlined its plans for renegotiation followed by an in-out vote ( i.e. a referendum giving options only of leaving and of remaining in under the current terms , or under new terms if these had become available ) , were the party to be re-elected in 2015 . The draft Bill stated that the referendum had to be held no later than 31 December 2017
2012 NATO summit meeting
2012 Chicago summit
2016 United Kingdom European Union membership referendum