The Prime Minister was elected on a majority vote of the Members of Parliament. Nothing changes automatically should she lose her majority. However, subject to the rules of the house, she could be subject to a vote of no confidence. Tradition suggests that should she lose such a vote she is compelled to visit the Queen and ask her to dissolve Parliament. She would then call elections.

Depending on the circumstances the Queen could ask another party leader to attempt to form a majority and if so, said leader would become the next PM