What is a treaty?
A treaty is a written agreement made between two or more states that is governed by international law.
In the UK it is the government that negotiates and signs treaties. Once signed, the document must be scrutinised by Parliament to confirm that the UK agrees to be bound by it. Only then may the treaty be ratified (by the government formally notifying the other parties that the UK will be bound by it from a specified date).
Ratifying a treaty
The Constitutional Reform and Governance Act 2010 (sec 20) requires the government to lay most treaties for parliamentary scrutiny for 21 sitting days.
If no action is taken by Parliament during the 21 day period, the government can proceed to ratify the treaty.
If, within the 21 day period either House decides that the treaty should not be ratified, the responsible Minister must issue a statement setting out why they believe the treaty should nevertheless be ratified. The statement resets the 21 day scrutiny period. There is no limit to the number of times this procedure can take place.