Opportunities were lost to tackle one of Britain’s largest grooming rings when information about two vulnerable girls being sexually exploited was not followed up, a review has found.

A report into the cases of more than 20 girls in Huddersfield found that social workers had evidence to conclude that two teenage victims were at risk of serious harm, yet took no action.

Dr Mark Peel, the author of the review, said children’s services officers knew that the two 16-year-olds, one of whom had learning difficulties, were “most likely to have been engaged in inappropriate, exploitative and illegal sexual activity” with older men.

He added: “In both instances, however, it would appear that, other than recording this information, no subsequent preventative safeguarding action was taken, and that thus an opportunity to break the [child sexual exploitation] ring operating in Kirklees, and to protect these girls directly and others more generally, was lost.”

The report, published on Monday, examined the files of 22 girls involved in the prosecution of 20 men who were given lengthy jail sentences last year after being convicted of a “vile and wicked” campaign of abuse spanning from 2004 to 2011.

Three trials heard how 15 vulnerable girls, aged as young as 11, fell victim to a grooming ring and were plied with drugs and alcohol before being subjected to abuse that was “at the top of the scale” of severity.

Peel, a social work academic, said that nothing relating to child sexual exploitation [CSE] was flagged up in the majority of cases, but that this may have been because social work professionals were not looking for CSE at that time.

However, he found that in the cases of two girls, referred to as Girl 4 and Girl 8, there was intelligence that they were at risk of serious harm. A reference to Girl 4 in one of the files said: “[Girl 4] has a moderate learning difficulty. She is being exploited into prostitution, she hangs around with a number of men who take her money. She is a very promiscuous girl.”

Concluding that “no action of any kind was taken in both these instances”, Peel said this “seeming lack of concern” for vulnerable children “now seems almost alien”. What social workers now understand as CSE was in 2006-07 perhaps seen as “nothing out of the ordinary” for young women typically on social work caseloads, he said, adding that “a different and lower safeguarding standard was implicitly applied to some young women in need of help and support”.

Mel Meggs, Kirklees council’s director of children’s services, said: “Dr Peel has been clear that the vast majority of cases were handled in line with the policy and practice of the time.

“However, the studies of Girl 4 and Girl 8 show that, historically, professionals did not always spot the signs of exploitation and did not always respond appropriately to concerns. We are truly sorry that these two girls were not protected in the right way.”