The Internet Watch Foundation (IWF) said it had received an "unprecedented" 191 reports of "sextortion" in the first half of this year, compared to 30 reports across the whole of last year.

The number of reports that saw action taken rose by 257% in the same period - and at least 6% of that content involved the most serious Category A images, the IWF said.

Sextortion is a form of blackmail in which a child is tricked into sending sexual images of themselves to abusers, who then threaten to share the pictures with friends, family or more widely on the internet if they are not paid money.

The IWF has said teenagers, predominantly boys, are most likely to be targeted on social media platforms. Adult abusers trick victims into believing they are talking to a young person of the opposite sex before convincing them to send an explicit image of themselves.

The charity's warning came as the government's Online Safety Bill faces its final stages in the Houses of Parliament this week - and campaigners have written to the heads of technology companies to demand they make online services safe for children.

Security minister Tom Tugendhat said: "We are working closely with the IWF and international partners and investing in new capabilities to enhance law enforcement's response to this specific threat. But we also need tech companies to do their bit."

The minister said he had written to Meta boss Mark Zuckerberg regarding the "deeply troubling" statistics, as the abusers often target their victims on social media platforms.

He said he urged Zuckeberg to "ensure child safety is upheld whilst rolling out end-to-end encryption on Instagram Direct and Facebook Messenger".

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'Significant concern'

The IWF's chief executive Susie Hargreaves OBE said: "It is shocking to see that more children are being cynically targeted in this way by manipulative abusers online.

"Blackmail is a serious offence, and a matter for the police, and children or adults who fall victim to this kind of abuse should contact local law enforcement."

The charity has said that on top of offenders sometimes being successful in their attempts to extract money from victims, the emotional impact of the practice can lead children to self-harm and take their own lives.

Ian Critchley, the National Police Chiefs' Council's lead for child protection and abuse investigations, said: "This data is deeply disturbing; showing a significant rise in the appalling and cynical way criminals seek to make money from abuse and coercion, with no thought for the life-long harm it causes these children and young people."

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Wendy Hart, deputy director for child sexual abuse at the National Crime Agency (NCA), said it had seen an increase in sextortion cases and it is "an issue of significant concern".