Deputy leader

A deputy leader (in Scottish English, sometimes depute leader) in the Westminster system is the second-in-command of a political party, behind the party leader. Deputy leaders often become deputy prime minister when their parties are elected to government. The deputy leader may take on the role of the leader if the current leader is, for some reason, unable to perform their role as leader. For example, the deputy leader often takes the place of the party leader at Question Time sessions in their absence. They also often have other responsibilities of party management.

...
Wikipedia