<P> Although it does not formally elect the prime minister , the position of the parties in the House of Commons is of overriding importance . By convention , the prime minister is answerable to , and must maintain the support of , the House of Commons . Thus , whenever the office of prime minister falls vacant , the Sovereign appoints the person who has the support of the House , or who is most likely to command the support of the House -- normally the leader of the largest party in the Commons , while the leader of the second - largest party becomes the Leader of the Opposition . Since 1963 , by convention , the prime minister is always a member of the House of Commons , rather than the House of Lords . </P>
the second largest party in the house of commons is usually known by what name
Leader of the Opposition