Instruments of Ratification

Signing of Accession Treaty (Photo: European Commission)
For an international treaty to be legally binding upon a state it must be signed by the head of that state or government, and it must be ratified. In a letter sent by the head of state of a particular EU member, he/she informs that the state in question has ratified an EU Treaty. The letter is sent to the Italian Government for historical reasons - the first treaties were signed in Rome.
The deposit of the letter or "instrument of ratification" is the final stage of the Treaty ratification process.
Once a letter of ratification has been sent there is no turning back. EU treaties come into force on an agreed timescale once letters (instruments) of ratification have been deposited by all Member States.
Since there is no legal provision for withdrawal from the EU at present, a state can only leave the EU - de facto - by its own decision or by unanimous decision of all Member States.

The future
According to the EU Constitution, it must be ratified by all Member States. If after 2 years a few Member States still haven't ratified it, the European Council has to deal with the issue. The Convention on the Future of Europe also proposes a clause allowing Member States to leave the EU after their own decision (Art. I-59). The Member State shall notify the Council of that decision and conclude a withdrawal agreement. If no agreement is reached, the membership shall cease 2 years after the notification. Meanwhile, the future Constitution remains in force. This means that if a Member State declares its wish to leave the EU, EU law would prevail over its national provisions for a period of up to 2 years.

See also Ratification.