Her Majesty's Opposition is usually the largest minority party in the House of Commons.
This is the party that would be most likely to form a government if the sitting one resigned.
The opposition sits to the Speaker's left in the Commons chamber, facing the government benches.
MPs from other political parties who do not sit on the government's side in the Commons also form part of the general opposition. This does not mean, however, that they will always vote with the official opposition and against the government.
In the same way that the Prime Minister forms a cabinet, so the leader of the opposition forms a shadow cabinet, which acts as a "government-in-waiting" and consists of the leading figures from the official opposition.
There are times, however, when it is unclear which party is the official opposition. This was the case between 1918 and 1922 when Labour and Liberals contended for the role in the face of a coalition government.