The Health Secretary said web giants were acting "not a moment too soon" – and insisted tough new laws are needed to keep them in line.
He said that "asking politely is not enough" as he vowed the Government will force firms to take responsibility for users' safety.
Yesterday Instagram finally moved to ban graphic images of self-harm after the tragic suicide of Molly Russell, 14.
She took her own life after being exposed to a flood of horrific pictures promoting suicide and self-harm.
But the firm, owned by Facebook, hasn't said when the ban will come into effect.
Mr Hancock said that Instagram and other large companies such as Google and Pinterest were "shocked and appalled" by the content found on their services.
He told Sky News: "Until now, what's happened is people have called for changes and politicians have asked the social media companies to make changes.
"Asking politely is not enough, we need to ensure that there's enforcement so society as expressed through Government can judge what is and isn't right on social media.
"We're not powerless, we can and we will legislate. What you can do is require them to act in a socially responsible way."
Asking politely is not enough
Speaking to the BBC, he added: "It's every parent's modern nightmare that their child will go on to Instagram and see the sort of material that encourages self-harm and encourages suicide.
"Social media companies shouldn't be the arbiters of this, it's for society to know what is and isn't appropriate.
"They've had a blind spot towards this material that promotes self-harm and suicide and they need to deal with it."
Labour deputy leader Tom Watson also joined the chorus of criticism, saying: "Too late to save the life of a 14-year-old girl, the boss of Facebook-owned Instagram is shamed by public opprobrium into changing his company's policy.
"But still no word from Mark Zuckerberg. Is he too cowardly, or too arrogant to speak? Or simply both?"
And NSPCC boss Peter Wanless added: "It should never have taken the death of Molly Russell for Instagram to act.
“We cannot wait until the next tragedy strikes. The Government must legislate without delay and impose a duty of care on social networks, with tough punishments if they fail to protect their young users.”
Ministers have promised to announce new laws cracking down on the web giants within weeks.
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