SNP bosses have been urged to block any attempt by Patrick Grady to return to the party after the Metropolitan Police dropped their probe into allegations of sexual harassment.
The Glasgow MP, who resigned his SNP membership late last month, was found by an independent investigation to have inappropriately touched a party worker.
But police have confirmed they will take no action against Mr Grady.
He was temporarily suspended from parliament in June after a Westminster ruling found him guilty of sexual misconduct at a pub in 2016.
SNP Westminster leader Ian Blackford faced calls to quit for backing him but Scotland Yard has confirmed their investigation has now been dropped.
Lib Dem chief whip Wendy Chamberlain – a former detective constable with Lothian and Borders police – said that the lack of a criminal investigation into the disgraced MP “does not mean that what he did was okay”.
She described Mr Blackford as a “moral vacuum” and called on him to resign as the SNP’s most senior politician at Westminster.
Ms Chamberlain said: “The SNP have been forced into allowing allegations of sexual misconduct within the Conservative party to pass without comment because they know they have failed at dealing with their own complaints.
“Just because the police have said they will not be conducting a criminal investigation into Patrick Grady does not mean that what he did was ok.
“Ian Blackford has been exposed as a moral vacuum, more concerned with protecting his friends than ensuring a young staff member got justice.
“Handling these types of allegations is something that all parties need to do better on.
“For the SNP, that should start with refusing to hand back the party whip to Patrick Grady and removing Ian Blackford from a position of leadership.”
The complainant against Mr Grady insists he has not received proper support from the SNP since speaking out and says the ordeal has been a “living hell”.
In a leaked recording, Mr Blackford was heard saying MPs should give Mr Grady their full support when he returned from his suspension.
The SNP Westminster boss apologised for the handling of the scandal, but was accused by rivals of trying to “save his own skin”.
Party colleague Amy Callaghan also issued an apology after she was heard encouraging colleagues to support Mr Grady in the recording.
Mr Grady, the SNP’s former chief whip, said he was “profoundly sorry” after the independent panel’s investigation.
A spokesman for the Met said: “On Wednesday June 22 police received a third party allegation of sexual assault that is said to have taken place in October 2016 at a commercial premises in Folgate Street, E1.
“Officers carried out enquiries including speaking to the alleged victim.
“There will be no further police action at this time.”
The SNP has been approached for comment.