Mhairi Black has urged Douglas Ross to "grow a backbone" and whip Scottish Conservative MPs to vote in favour of sanctions against Boris Johnson.
The former prime minister was yesterday found by a damning report to have knowingly misled Parliament over Partygate.
Johnson could now not receive the pass for Westminster usually given to ex-MPs.
Scots Tory leader Ross, who sits as an MP as well as an MSP, has been repeatedly criticised for performing a u-turn on his call for Johnson to stand down last year.
Black, the SNP depute Westminster leader, today called on him to whip Scottish Tory MPs to vote in favour of sanctions against Johnson.
She said: “Douglas Ross must finally grow a backbone and whip his MPs to vote for tough sanctions against his disgraced former leader Boris Johnson.
“It’s well known that Mr Ross lacks authority with Scottish Tory MSPs and MPs, who are discontent with his poor leadership.
“He might be desperate to avoid a leadership challenge but failing to whip his MPs will only make him look even weaker.
“Mr Ross can’t hide from the fact that he and his Tory colleagues put a serial liar into Number 10 – and repeatedly backed Johnson, despite all the damage he inflicted on Scotland.”
A spokesman for the Scottish Tories said: “Douglas has made his views clear and will vote in favour of the committee’s findings. As it’s a free vote, it’s up to other MPs how they choose to vote.
“SNP MPs should look to their own party in Holyrood, where MSPs are being forced to back Nicola Sturgeon.”
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