Under the UK’s constitutional arrangements, there is no requirement to hold a referendum on the ratification of an EU Treaty. Indeed previous Governments have not sought to hold such a referendum. It is for Parliament to pass the necessary law.
In certain cases the Government have recommended that particular arrangements should be subject to a referendum. It is for Parliament to decide whether to hold such a referendum, having regard to the particular circumstances of the case.
In the case of the Treaty Establishing a Constitution for Europe I refer the hon. Member to the statement by my right hon. Friend the Prime Minister to the House on 20 April 2004, Official Report, columns 155-57.