This question was answered on 21st March 2016
Decisions on accession to the EU are subject to strict conditionality, and made on a case by case basis. All new accessions are subject to agreement by all 28 Member States at every step of the negotiations. If and when, at the end of successful negotiations, Member States have unanimously agreed to admit a candidate country to membership, the resulting Accession Treaty must be ratified by each Member State in accordance with their own respective national arrangements, and also approved by the European Parliament. In the case of the UK, the EU Act 2011 requires any Accession Treaty to be ratified by Act of Parliament.