Speaking at the Global Progress Action Summit, Sir Keir warned against accepting defeat to populism and emphasised standing up for values of decency and honesty.
A recent YouGov poll indicates Reform UK could secure 311 seats, becoming the largest party in a hung parliament and significantly increasing Nigel Farage's chances of entering Downing Street.
Sir Keir is facing internal pressure, with his personal approval ratings low and Andy Burnham openly positioning himself to replace him as Labour leader.
The Prime Minister declared the next election will be an "open fight" between Labour and Reform for the "soul of the country" proposing digital ID cards to tackle illegal migration.