Since the emergence of the lobbying scandal involving ex-PM David Cameron it seems there have been new allegations levelled at Conservative politicians almost daily.
Sir Keir is likely to grill Mr Johnson over how the renovation of his Downing Street flat was paid for, after ITV News exclusively reported the Tory party made a payment to the Cabinet Office to cover initial costs of work - which the prime minister is now repaying.
Another line of attack for the Labour leader will be the allegations that Mr Johnson remarked last year that he'd rather let "bodies pile high in their thousands" rather than order a new lockdown. He has denied using those words.
Last week's attacks at Prime Minister's Questions - over the PM's texts to billionaire James Dyson about tax breaks for his firm - failed to land any severely damaging blows because Mr Johnson refused to apologise for "doing everything I could... to secure ventilators for this country".
Will allegations of Tory sleaze stick? Listen to the Calling Peston podcast:
But Mr Johnson may find it more difficult to bat away new criticisms, especially since two witnesses have told ITV News they would "speak under oath" to confirm he did make the "bodies pile high" comments.
Sir Keir may also use his time at PMQs to ask about NHS waiting times, given 4.7 million people are waiting for treatment - the highest since records began in 2007.
Both leaders may acknowledge International Workers' Memorial Day by commending those who have died while leading the fight against coronavirus.
Watch the full session of PMQs at the top of this page from midday.