The Independent reports that Dr. Andrew Watt plans to go to court to see if any agreement replacing the Lisbon Treaty as a result of the UK’s Article 50 notice requires a referendum under the European Union Act 2011. The parliamentary draftsman carelessly states that a treaty or decision amending the “Treaty of the European Union” cannot be ratified unless there’s a referendum. In the pre-amble, it states that amendments include amendments taken under Articles 48 & 49, but it’s not exclusive i.e. other amendments to the Treaty also trigger the Law. Part 1 §4 lists the type of changes which must trigger a referendum and they do not include any arrangements on leaving, but they do include “extensions of competence”, new obligations (for the UK) and new sanction powers for the EU.
I would argue that any transitional treaty which does not extend time of the UK’s rights to appoint a Commissioner, appoint Judges to the CJEU and elect MEPs will require a referendum and other clauses may also do the same. Even if some disagree, there can be surely no doubt that it will require a statute mandated Parliamentary statement and an Act of Parliament and it’s the Act of Parliament that determines if there should be a referendum.
This could be one reason why they are so keen to be thrown out.
Any treaty amending the EU Treaty, must be considered as potentially requiring a referendum. Any such treaty must be considered by Parliament and approved by an Act of Parliament. (Both Houses, four readings). If the Government and Parliament consider that the new treaty does not breach any §5 clauses, a referendum is not required, otherwise it is and must be passed.