Ian Blackford was accused of trying to “save his own skin” as he apologised for his mishandling of the Patrick Grady scandal.
The SNP Westminster chief faced further demands to quit after he backed Glasgow MP Mr Grady, who inappropriately touched a party staff member then aged 19 in 2016.
Mr Grady was temporarily suspended from parliament after an independent parliamentary panel found him guilty of sexual misconduct.
Labour said Mr Blackford’s statement was “not a real apology” and the Tories insisted he has no credibility left, while the complainant branded his apology a “publicity stunt”.
But in a leaked recording, Mr Blackford urged MPs to offer “full support” to their colleague and said he looked forward to welcoming him back.
While the House of Commons SNP leader admitted Mr Grady’s behaviour was “unacceptable”, he did not directly apologise for backing him.
Instead he apologised that the recording leaked from a Westminster meeting had caused the victim “distress”.
The SNP Highland MP said: “I deeply regret that a member of staff was subject to inappropriate behaviour. It was completely unacceptable and should never have happened.
“Staff have a right to feel fully supported when a complaint is made. I regret that the complainant does not feel that this is the case.
“The way this situation has played out publicly over the past few days, including recordings from the parliamentary group, has caused distress to the complainant and I am sorry that this is the case.”
Mr Blackford also announced that an independent probe will be held into support available for staff who have complaints.
But he showed no indication that he will bow to pressure and resign.
‘Not a real apology’
Labour MSP Neil Bibby said: “This is not a real apology, it is the apology of someone who got caught.
“From the start, the SNP have behaved in a way that is disgraceful.
“Mr Blackford is leader of the UK’s third party and a close ally of the First Minister, he must stand aside.”
Tory MSP Craig Hoy said: “It’s astounding that it took Ian Blackford this long to realise he should apologise.
“The SNP Westminster leader has shown appalling judgement and is only now saying sorry in a desperate bid to save his own skin. His credibility is in tatters and he must go.”
Mr Blackford’s apology came after East Dunbartonshire MP Amy Callaghan said she was sorry for offering Mr Grady her support in the recording.
She admitted her comments were “insensitive” and added that she was “deeply sorry”.
Senior SNP minister Angus Robertson, who was party Westminster leader before Mr Blackford took over, insisted his successor should not have to quit.
Mr Grady remained in his post as chief whip after the accusations against him first surfaced, but then stepped back last year.