Following a General Election, all MPs and peers must take an oath of allegiance to the Monarch before they can take part in the work of Parliament.
In the Commons, the Speaker is the first to take the oath, followed by members of the government frontbench, members of the opposition frontbench and other Privy Councillors.
This process usually takes several days and can extend into weeks if for any reasons an individual MP cannot attend at the opening of the session.
MPs who, for religious or other reasons, do not want to take the oath, can instead affirm.
Winners of by-elections follow the same procedure when they take their seat in the chamber.
The Oath and Affirmation
To take the oath, a member holds a copy of the New Testament, the Old Testament or the Koran and says: "I [name] swear by Almighty God that I will be faithful and bear true allegiance to Her Majesty Queen Elizabeth, her heirs and successors, according to law. So help me God."
The affirmation is worded: "I [name] do solemnly, sincerely and truly declare and affirm that I will be faithful and bear true allegiance to Her Majesty Queen Elizabeth, her heirs and successors according to law."