The SNP has launched an external review into the support available to staff after an MP accused her party of “clearly falling short” of supporting sexual harassment complainants.

It follows the party’s former Westminster chief whip Patrick Grady being suspended from the SNP’s Westminster group for a week, as well as being suspended from parliament for two days, over a sexual advance towards a teenage staff member in 2016.

The party’s Westminster leader, Ian Blackford, said he regretted that the complainant did not feel fully supported. He faced calls to resign this week after an audio recording in which he could be heard apparently encouraging colleagues to “give as much support as possible” to Grady was leaked.

However, he has now said that Grady’s behaviour was “completely unacceptable” and “should never have happened”. “The way that this situation has played out publicly over the last few days, including recordings from the parliamentary group, has caused distress to the complainant amongst others and I am sorry that is the case,” he said.

“We will consider all lessons that must be learned to make sure staff have full confidence they will receive the support they need. As such, I am initiating an external review of support available to staff, to sit alongside the independent advice service and independent complaints process.”

The announcement came as the East Dunbartonshire MP, Amy Callaghan issued a “wholehearted apology” after she was heard on the tape also appearing to support Grady, while failing to express sympathy to the teenage victim. “Zero tolerance can’t be a slogan, it has to be real,” she said on Tuesday.

Grady, the Glasgow North MP, told the Commons last week that he was “profoundly sorry” after the independent expert panel, which recommends punishments for MPs over bullying, harassment or sexual misconduct, found he had touched and stroked the neck, hair and back of a colleague during a social event.

Callaghan unseated the then Liberal Democrat leader, Jo Swinson, in the 2019 general election and is considered a protege of the first minister, Nicola Sturgeon. Her apology came as Scottish Labour and the Conservatives both called for Blackford’s resignation, after the Daily Mail obtained a recording of the SNP’s Westminster leader urging fellow MPs to give “as much support as possible” to Grady.

Grady’s victim, who is still employed by the party but has been signed off work, said after the recording emerged that it would be extremely difficult to return to work and is now considering legal action.

The victim, now 25, has previously described facing exclusion and “bullying” after they came forward with their initial complaint, saying: “I thought the SNP was a party of equality but after working in Westminster for six or seven years I can see now that they’re not any different.”

Callaghan’s intervention adds momentum to accusations of hypocrisy over how the party handles such complaints. Critics have compared Grady’s treatment with that of the MSP Mark McDonald, who resigned as children’s minister and was suspended by the party after he was found to have sent inappropriate messages to women, and Derek Mackay, who resigned as finance secretary and was also suspended from the SNP after sending hundreds of messages to a 16-year-old boy.

Last year, the party was convulsed by a lengthy Holyrood inquiry into the Scottish government’s botched handling of sexual harassment allegations after former first minister Alex Salmond was cleared of sexual assault charges.

A woman who made sexual assault allegations against Salmond later described the Holyrood inquiry as “in many ways more traumatic” than the trial itself.

In a statement posted on Twitter on Monday, Callaghan said she took “full accountability for the hurt and disappointment I’ve caused, not least of all to those directly impacted by sexual misconduct in this case”.

On the recording, Callaghan reportedly tells her fellow SNP MPs at last week’s group meeting “we should be rallying together around him to support him at this time”.

She explained in the statement: “I believed I was in a situation where my support of survivors was implied. I was wrong. This isn’t good enough.”

Callaghan added that she had written to the Westminster party’s chief whip calling for a “root and branch review, commissioned by an independent external organisation, of our internal misconduct and harassment structures”.

Earlier in the week, the MP Joanna Cherry, who has been an outspoken critic of Blackford’s leadership style, posted on Twitter: “I wasn’t at the SNP Westminster group meeting last week. I don’t condone the covert recording or leak. However, for some time the SNP has had significant problems in how it handles complaints.

“My party needs to reflect on the contrast between the treatment of different ‘offenders’ and to review our arrangements for the pastoral care of complainers.”

A spokesperson for the SNP Westminster parliamentary group said it accepted the recommended actions from the independent expert panel: “We welcome Mr Grady’s apology and note that he previously apologised for this incident when the matter was dealt with informally in 2018.”