It should be underlined that granting this independence is, as it should be, a political decision. An independent central bank needs a clear legal mandate. The ECB is one of the most, if not the most, independent central bank in the world.
Its independence and that of the participating national central banks is firmly enshrined in the so-called Maastricht Treaty, the Treaty on European Union. This Treaty was concluded by the governments of all the EU Member States and ratified by all their national parliaments. In some cases referenda were held.