LONDON (Reuters) - Keir Starmer, leader of the opposition Labour Party and favourite to become Britain's next prime minister, said on Friday he would quit if his party failed to win the country's July 4 election.

Asked during a BBC phone-in programme whether he would resign if Labour lost or lost heavily, Starmer said: "Yes."

Labour is around 20 percentage points ahead in most opinion polls, and forecast to comfortably win enough parliamentary seats to form the next government, ending Prime Minister Rishi Sunak's tenure and 14 years of Conservative government.

(Reporting by William James, editing by Elizabeth Piper)