Johnson is facing the gravest threat to his premiership over alleged lockdown-busting parties at his Downing Street residence and office, some of which are being investigated by the police, but has so far weathered growing calls to resign.
Asked if Johnson should resign if it emerges he lied to parliament, Truss, considered one of the frontrunners to succeed him as leader, told BBC Television: "I'm not going to answer hypothetical questions. The future of the prime minister is assured."
(Reporting by Kylie MacLellan; editing by David Evans)
