It comes after a government-requested audit into the scale of grooming gangs across the country concluded a nationwide probe was necessary.
The prime minister previously argued a national inquiry was not necessary, but has changed his view following an audit into group-based child sexual abuse led by Baroness Casey, which is set to be published next week.
“[Baroness Casey’s] position when she started the audit was that there was not a real need for a national inquiry over and above what was going on,” he told reporters travelling with him to the G7 summit in Canada.
“She has looked at the material… and she has come to the view that there should be a national inquiry on the basis of what she has seen.
“I have read every single word of her report, and I am going to accept her recommendation. That is the right thing to do on the basis of what she has put in her audit.
“I asked her to do that job to double check on this; she has done that job for me, and having read her report… I shall now implement her recommendations.”
Grooming gangs timeline: What happened, what inquiries there were and how Starmer was involved
The near 200-page report is to be published next week and is expected to warn that white British girls were “institutionally ignored for fear of racism”.
One person familiar with the report said it details the institutional failures in treating young girls and cites a decade of lost action from the Jay Review, set up in 2014 to investigate grooming gangs in Rotherham.
The report is also expected to link illegal immigration with the exploitation of young girls.
The government had initially resisted a new inquiry, insisting that it first wanted to implement recommendations from previous inquiries, such as the Jay Review, into child sexual exploitation.
It also allowed five councils to set up their own investigations into their communities rather than hold a national inquiry.
The grooming gang scandal came back into the headlines at the beginning of the year after Elon Musk attacked Sir Keir and safeguarding minister Jess Philips for failing children.
The prime minister and Ms Phillips hit back, with Sir Keir citing his record of prosecuting abusers as director of public prosecutions, while Ms Phillips has long been a campaigner against domestic violence.
At the time, she told Sky News that Mr Musk’s claims were “ridiculous” and that she would be led by what victims have to say, not him.
Following the row between the tech billionaire and the UK government, the prime minister asked Baroness Casey to conduct an audit of all the evidence to see if a national inquiry was required.
Read more on this story:
Telford child abuse victims speak out
What we know about grooming gangs, from the data
The women who blew whistle on Rotherham
Kemi Badenoch, the Conservative leader, said: “Keir Starmer doesn’t know what he thinks unless an official report has told him so.
“Just like he dismissed concerns about the winter fuel payment and then had to U-turn, just like he needed the Supreme Court to tell him what a woman is, he had to be led by the nose to make this correct decision here.
“I’ve been repeatedly calling for a full national inquiry since January. It’s about time he recognised he made a mistake and apologise for six wasted months.
“But this must not be the end of the matter. There are many, many more questions that need answering to ensure this inquiry is done properly and quickly.
“Many survivors of the grooming gangs will be relieved that this is finally happening, but they need a resolution soon, not in 10 years’ time. Justice delayed is justice denied.”
