The coronavirus lockdown has created a "perfect storm" for many children isolated with their abusers, ex-home secretary Sajid Javid has said.
Writing in the Telegraph, he said this will contribute to a "surge" in cases.
He said he will lead a new "no holds barred" inquiry into child sex abuse in the UK with the Centre for Social Justice think tank.
The inquiry will examine organised child sexual exploitation and the abuse of children online.
It comes as the NSPCC says its helpline for adults has responded to more than 10,000 "child welfare contacts", including calls and emails, since the start of the UK's lockdown in March.
Concerns included emotional abuse, sexual abuse and neglect and parental mental health.
Meanwhile, Home Secretary Priti Patel announced last month that the government will publish a paper "later this year" on research into group-based child sexual exploitation, which was commissioned by Mr Javid when he was home secretary in 2018.
Mr Javid told BBC Radio 4's Today programme that something that "weighed the most heavily on him" during his time as home secretary in 2018 and 2019 was child sexual abuse and its "true scale".
He said he was "particularly concerned" about lockdown because "children are left to isolate alongside their abuser and they will therefore suffer severe long-term damage and this kind of thing isn't reflected in statistics just yet, but it will be, and I'm very concerned about that".
The former chancellor said the investigation into will look at organised child sexual exploitation, including gangs and on-street grooming.
The second part of the inquiry will examine how child sexual abuse "happens today", with a focus on online abuse and live streaming.
'Saddened and angered'
Of the gang-based exploitation, Mr Javid said: "We know that of all these high profile cases when there have been convictions, a disproportionate number of people are from Asian heritage, particularly Pakistani heritage, my own heritage and that both saddens and angers me.
"People from my heritage, many of them disproportionately responsible for what we've seen and I want to know know why."
He said in the past there had been an "ignorance" of this in some authorities.
Writing in the Telegraph, Mr Javid said: "The surge in child sexual abuse happening right now won't be reflected in statistics until later this year.
"As appalling as those numbers will be, however, they'll still only scrape the surface of what's been occurring under our noses for decades."
Andy Cook, chief executive of the Centre for Social Justice think tank, said it was "highly courageous" of Mr Javid to "speak out on the issue, which has been difficult to confront and too often neglected".
Anna Edmundson, the NSPCC's head of policy and public affairs, said children were the "hidden victims of the crisis".
"They have been exposed to increased risks of abuse while having limited access to those who can keep them safe because so much of life has been behind closed doors," she said.
"Abusers have also been grasping an unprecedented opportunity to target children online since tech firms have had to scale back on moderators and young people have been spending more time on the internet, with many feeling increasingly isolated and lonely."
She said the "true scale" of abuse will not be known "until later down the line" and any concerns can be reported to the NSPCC's helpline.
Barnardo's chief executive Javed Khan said the charity had "consistently warned" that the coronavirus lockdown was "putting children at increased risk of sexual abuse and exploitation" and so the investigation was "very welcome".
"Few of us can imagine what it feels like for victims trapped in an unsafe home, isolated from support systems, school and friends, and unable to get any respite from their abuser. But this is the experience for many children today and over the last few weeks," he said.
"With both children and abusers spending more time online during the lockdown, they are also at increased risk of being targeted by strangers through social media, apps and gaming.
"This investigation should shed light on children who have been 'hidden' from professionals during the lockdown, and explore how they can be identified and access the support they need."
In 2018, in his role as home secretary, Mr Javid ordered research into the "characteristics and contexts" of gangs abusing children, arguing that ignoring issues such as ethnicity is more likely to fuel the far-right.
He said he wanted officials researching the causes of gang-based exploitation to leave "no stone unturned".
Due to be published later this year, the paper on this review "will outline the insights gained" and will "focus on how agencies can learn lessons from the past to tackle group-based offending and safeguard vulnerable children".
Information and support: If you or someone you know needs support for anyone affected by sexual abuse, these organisations may be able to help.