Several shadow ministers defied the Labour leader last month by joining rail protests – after which he backed down on disciplinary threats.
But Sir Keir waded back into controversy ahead of a fresh rail strike and the Royal Mail dispute.
He said the Labour party in opposition “needs to be the Labour party in power”, adding: “A government doesn’t go on picket lines, a government tries to resolve disputes.” But his remarks put him on a collision course with the Hard Left.
Leeds MP Richard Burgon said: “The strikes are part of the fightback against our rigged economy.”
“If the strikers win, all workers win.”
Left-winger Zarah Sultana also expressed her support for the latest protest while ex-shadow chancellor John McDonnell will join union members today at an online event called Building the Resistance: Unity through Struggle.
Sir Keir also told Good Morning Britain it would be up to a Labour government to “create the framework for success” in talks, rather than just dish out inflation-matching public sector pay rises.
He also faced a further backlash from the Left as he revealed he had abandoned 10 “socialist” pledges he made when standing to be party leader, including a vow to nationalise vast swathes of the economy.