LONDON — Under lashing rain, Prime Minister Rishi Sunak on Wednesday said Britons would be heading to the polls on July 4 — the first general election in years with a high probability of ousting the long dominant Conservative Party and ushering in a new era of Labour Party leadership.

Speaking at a lectern outside 10 Downing Street, his dark tailored suit soaked, the prime minister said: “Now is the moment for Britain to choose its future. To decide whether we want to build on the progress we have made, or risk going back to square one with no plan, no certainty.”