Two girls were raped in Fordingbridge, Hampshire, by two 15-year-olds who were given non-custodial sentences by a judge at Southampton Crown Court who said he wanted to "avoid criminalising these children unnecessarily".
But one of their victims told the BBC: "The words hit like a rock straight in my face".
"He (the judge) almost made it seem as if what the boys did was not OK, but it was OK in the eyes of the law because they were still children," she said.
In a post on X responding to the interview, Starmer characterised it as "harrowing and brave testimony".
"The girls at the heart of this case have shown extraordinary bravery and strength in heinous circumstances," he said.
"This is an appalling case and it is right that law officers are urgently reviewing the sentences."
Cabinet minister Darren Jones had earlier said the girls "deserve justice".
"And quite frankly, other boys need to know that they can't behave in that way and get away with it," he said.
He suggested a review of the sentences by the Attorney General under the unduly lenient scheme would be completed quickly.
Attorney General Lord Richard Hermer has 28 days from the date of the sentencing hearing, or until June 18, to refer the case to the Court of Appeal, but Jones said he would make a decision more quickly.
There has been public outcry over the sentences the boys were handed, with both being given youth rehabilitation orders and being made subject to intensive supervision and surveillance.
Jodie Mittel KC, prosecuting, told the trial that one of the girls, who was 15 at the time, had visited one of the defendants after meeting him on Snapchat.
The prosecutor said that after performing sex acts on the boy, who was then 14, she became "scared and anxious" when the second defendant arrived, and the pair raped her while the incident was filmed.
Mittel said that afterwards, videos of the incident had been sent around and other people made jokes about the girl, and she received messages calling her a "slag".
The girl told the BBC she "wanted to die" when she received such messages.
The second complainant was 14 when she was raped in a field near Fordingbridge recreation ground in January 2025, while the incident was also filmed.
In the sentencing hearing on Thursday, one of the 15-year-old boys was handed a three-year youth rehabilitation order with 180 days of intensive supervision and surveillance for the rape of each of the two girls and two indecent images charges.
The second 15-year-old was given the same sentence for three charges of rape against each of the two victims and four counts of taking indecent images in relation to filming of the incidents.
A third boy, 14, was given a youth rehabilitation order for 18 months for two charges of rape in the January incident, by encouraging the second defendant and an offence of indecent images.
Judge Nicholas Rowland told the defendants, "I have to remember that you are not small adults".
"I have to think how likely you are to do serious things again and I need to make sure you do not do serious things again in the future," he said.
He added that "peer pressure played a large part in what went on".
One of the girls' mothers issued an appeal to Starmer in the BBC interview.Â
"If it were your daughter, your niece, your son, your nephew, your family member, would you be happy?" she said.
