LONDON—The U.K.’s new chancellor, Jeremy Hunt, effectively scrapped Prime Minister Liz Truss’s signature tax-cutting plan, saying on Saturday that some levies would have to rise and public-spending cuts would need to be made to restore market confidence in the British economy.

Mr. Hunt said that Ms. Truss had made mistakes with her plan to boost growth by borrowing money to fund the biggest tax cuts since the 1970s, measures that spooked the markets and sent the Conservative Party crashing in the polls.