The internet giant was accused of allowing the vulnerable teenagers to be 'brainwashed in their bedrooms', Daily Mail reports.

It came as the straight-A pupils' distraught families begged them to come home amid fears they have already reached Syria after being radicalised on the social media site.

families begged them to come home amid fears they have already reached Syria after being radicalised on the social media site

And East London Mosque also issued an emotional plea for the missing schoolgirls to get in touch before asking worshippers to come forward with any information which could help police enquiries.

The Metropolitan Police last night confirmed officers were in Turkey but refused to say whether they were involved in the search for the girls.

One of the girls was 'following' more than 70 known extremists on the site – many of them thought to be ISIS fighters.

This would have exposed her to a stream of graphic material – including footage of beheadings and the mutilated bodies of children. But despite these accounts being well known, none have been suspended by Twitter bosses.

At least one of the girls is also thought to have been contacted directly using Twitter by a female ISIS fighter.

Yesterday their families made fresh appeals to the girls, begging them not to cross the border if they had not done so already.

'We are depressed, and it's very stressful. The message we have for Amira is to get back home. We miss you. We cannot stop crying. Please think twice. Don't go to Syria.

Reading a message to Kadiza, her older sister Halima Khanom said: 'We want you to know that we all miss you and we love you.

Renu Begum, 27, the older sister of Shamima, said: 'We love her, she's our baby. She's a sensible girl. We're hoping she wouldn't do anything that would put her in any danger.

'We're just holding on to hope that she hasn't gone to do anything stupid.'

Counter terrorist police in several countries remain locked in an increasingly desperate hunt for the girls.

Shamima and Kadiza were both prolific Twitter users who used privacy settings to mask their activities.

But messages recovered by the Daily Mail reveal Kadiza was recently linked with hate preacher Anjem Choudary.

The schoolgirl amassed almost 11,000 followers with her prolific messages, many praising life in territory controlled by ISIS.

Meanwhile Shamima followed 77 accounts, of which almost every single one belongs to a terrorist fighter or IS sympathiser.

They gave her access to a torrent of appalling images and footage.

Dozens of grisly accounts linked to ISIS terrorists who boast of atrocities and call for fresh attacks in Western states have been closed down by Twitter.

In one particularly graphic tweet, an Australian terrorists published a photograph of his seven-year-old son holding up a severed head. 

But Twitter's refusal to check new accounts mean they can simply spring up again under slightly different names within minutes.

Last night, smugglers in Akcakale, more than 800 miles from Istanbul, reported seeing the three girls crossing into Syria through the official border on Friday.

One said they had Syrian ID documents and were with a man who claimed to be their father. Counter terrorist police are scouring online communications by the girls in the hope of tracing their movements and potential accomplices.