Investigation was launched in the wake of a young party member's suicide
Thirteen people have been identified as being victims of alleged bullying, harassment or inappropriate conduct by a former Tory activist in the UK.
A report, compiled by law firm Clifford Chance for the Conservative Party, identified four cases that amounted to such alleged behaviour against young members that were reported before August 2015.
It comes after a scandal that resulted in the then International Development Minister and former Conservative Party chairman Grant Shapps being forced to resign.
Mr Shapps quit after claims he failed to deal with allegations of bullying within the party's youth wing.
The former activist, Mark Clarke, who denies any wrongdoing, was previously director of the organisation Road Trip, which was set up to bus Conservative activists to marginal seats during the 2015 General Election campaign.
Of those four cases that were reported, two were made verbally and the people to whom the complaint was made did not recall it. One was not considered to be a complaint about bullying at the time.
The scandal blew up after the suspected suicide of 21-year-old Conservative activist Elliott Johnson in September 2015.
Mr Johnson was found dead on railway tracks after making a complaint that he had been bullied by Mr Clarke.
Mr Johnson's father, Ray, called on Mr Shapps and Conservative Party Chairman Lord Feldman to quit, saying his son would still be alive if they had acted on the allegations.
It forced the then Prime Minister David Cameron to intervene, saying he would personally ensure there was a "proper investigation" into bullying allegations.
New training introduced
Emails sent to and from Grant Shapps examined by Clifford Chance showed he recognised there was a potential risk in incorporating Road Trip within the structure of the Conservatives' campaign.
This was because he was aware of Mr Clarke's reputation during the 2010 election campaign after he reviewed Mr Clarke's candidate file, the report said.
As a result, the report concluded, there was no evidence that Mr Shapps was aware of allegations of bullying or harassment of young activists by Mr Clarke or those associated with him prior to the 14 August 2015 complaint.
The report exonerated Mr Shapps' co-Chairman Lord Feldman, finding no evidence that he was aware of any such allegations at the time.
Lord Feldman was a key ally of the then Prime Minister, and was credited with helping deliver election success in May of that year.
The report said Elliott Johnson's parents were invited to take part in the inquiry but declined, telling Clifford Chance through their solicitors they "would neither support nor participate in the investigation because they had concluded that it did not appear to be independent or transparent".
The Conservative Party said that as a result of the report's findings, it had introduced a series of measures including; a dedicated complaints procedure for volunteers, a code of conduct for volunteer leaders, new training for team leaders and employees and strengthening reporting lines.