The Labour leader was understood to be in talks on Monday about whether to announce the pledge, after pulling out of a scheduled event where he would have faced fresh questions.
Allies insisted he was not “ducking scrutiny” by cancelling his attendance at the discussion, as he is investigated by police over the Durham beer-and-curry gathering in April last year.
A pre-emptive announcement on his future if he too is fined would enable him to continue calling for Mr Johnson to leave Downing Street while himself under investigation.
Sir Keir was set to make a televised statement at 4pm.
He had been due to speak at an Institute for Government discussion, but pulled out on Sunday, with Labour not explaining why other than to say “plans change”.
Sir Keir also did not attend a memorial service for former MP James Brokenshire at St Margaret’s Church in Westminster, where he was expected join politicians from across the divide including the prime minister and Cabinet members.
Labour says the food was consumed between work events, meaning it was within the rules despite the ban on indoor socialising.
But the Labour leader was facing calls to answer fresh questions after a leaked memo suggested the takeaway was planned, with no further work apparently scheduled after dinner.