Green Party members did not adequately follow up a report of child sex offence charges against one of its election agents, an investigation said.
David Challenor, 50, was charged in November 2016 but worked for his daughter Aimee Challenor who was a candidate at the 2017 general election.
The independent investigation has recommended the party urgently reviews its safeguarding policy.
The party said it is taking immediate action to implement necessary changes.
Challenor continued working for his daughter after being charged and also acted as her agent at local elections in May 2018.
In August 2018, Challenor, of Charterhouse Road, Coventry, was found guilty of 20 offences including the rape of a 10-year-old girl.
He held a girl captive in his attic and photographed the abuse, the court heard.
The investigation report, by independent consultancy Verita, said the nature of the charges raised issues about whether other children or vulnerable adults were at risk.
On 5 November, Ms Challenor sent a Facebook message to party communications co-ordinator Matt Hawkins and Clare Lorraine Phipps, chair of its national executive committee, referring to the charges adding “it is my understanding that the majority of them were sexual offences”.
But, it said, she did not ask them to take action, or mention her father was a party member.
The report found that although the pair had little information, they “closed the matter off too quickly and should have followed up more”.
It also said members it interviewed had little awareness of safeguarding issues and those who were told about the charges “saw the issue as primarily a communications one – about protecting the reputation of the party”.
It added: “Clearly the reputation of the party, and therefore political considerations, are at the forefront of the people’s minds.
“However, issues such as safeguarding also need to be prioritised.”
The report recommended the party should update its code of conduct, review safeguarding policy and speak to West Midlands Police about whether it should have drawn attention to the charges.
But the force said it does not appear Challenor’s involvement with the Green Party was ever disclosed to police.
A party spokesperson said: “We deeply regret that this situation arose and we apologise unreservedly for any concerns that this will have caused and any safeguarding risks it may have given rise to.”
Ms Challenor, 20, who had been running to become the Green’s deputy leader was suspended by the party pending the investigation and later resigned, accusing the party of transphobia.
She said: “Thankfully there is no evidence that anyone was close to harm as a result of their contact with the Green Party.
“My thoughts continue to be with those my father caused harm.”
She said she stands by her apology for appointing her father.
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