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London children under the age of ten are being “trained to be junior jihadis” in a disturbing new sign of the growing extremist threat in the capital, City Hall’s policing supremo warned today.

Deputy Mayor Stephen Greenhalgh said that the way some children were being radicalised was “horrendous” as he revealed that both he and Boris Johnson have been briefed on cases in which primary pupils have been subjected to propaganda and “extremist ideology” by their families.

His comments, ahead of a counter-terrorism summit at City Hall today, came as a new report added to the concerns about terrorism by warning that at least 40 British fighters involved in the conflict in Syria and Iraq are likely to attempt “blowback” terrorist attacks when they return to this country.

Mr Greenhalgh insisted that Londoners should not be “panicked” by the increased danger, but said that he was deeply concerned about the indoctrination of children and its potential to fuel the jihadi problem for years to come.

“The Mayor and I have seen case studies which are quite shocking,” he said. “Some of them are very young. We are talking about under the age of ten. It starts very young with them being subjected to propaganda, extremist ideology, and being trained to be junior jihadis.

“It’s pretty horrendous when you hear how some of these children are being radicalised. The threat of radicalisation of young people is real and this is a problem that is going to be with us not just for a couple of years, but for the next generation.”

Mr Greenhalgh, who oversees policing in the capital, said that he was unable to disclose further details of the children affected, but confirmed that they were from London and that the information had been provided in confidence by Scotland Yard and the intelligence agencies to allow him and the Mayor to understand more fully the impact of radicalisation on the young.

He said that to combat the problem City Hall wanted to ensure that teachers, social workers, and other child protection staff knew how to spot the signs of radicalisation and that the problem was treated as seriously as other forms of abuse.

He also warned of the need to be alert to Birmingham-style “Trojan Horse” plots targeting London schools and added that the problem of radicalisation was so severe that London was likely to face terrorist threat for a “generation” or more.

Mr Greenhalgh’ comments came as he hosted a counter-terrorism summit at City Hall attended by police and crime commissioners from other major cities, including Birmingham, Manchester and Leeds. The event was also attended by Scotland Yard counter-terror chief Commander Helen Ball and the government’s former terrorism watchdog Lord Carlile.

The aim of today’s meeting, which follows earlier warnings from the Prime Minister David Cameron and Met Commissioner Sir Bernard Hogan-Howe about the growing terror threat to this country posed by the conflict in Syria and Iraq, was to share methods used in each city for tackling violent extremism and preventing attacks.

Mr Greenhalgh said that a key aim of today’s meeting was to ensure that clear “referral paths” were provided for those who worked with children to report any case in which they suspected that a child was being indoctrinated with extremist ideology. He said other methods, incuding the use of “informants”, were also needed and that the radicalisation of children by adults was child abuse.

In a further warning, Mr Greenhalgh said that London schools also faced a “Trojan Horse” risk - in a reference to the recent row over an alleged Islamist takeover of some schools in Birmingham - and that advice on how to identify the “warning signs” would be prepared.

Today’s comments by the Deputy Mayor follows the arrest of several teenagers over alleged terror offences linked to the Syria and Iraq conflict. Another indication of the problem came earlier this year when a girl, aged no more than 12, was photographed carrying a black “jihadi” flag associated with al Qaeda on a London street.

Meanwhile, a new report published today warns that at least 40 Britons fighting in Syria and Iraq will seek to carry out “blowback” terrorist attacks in this country after their return.

The report, by the Henry Jackson Society think tank, says that there is “significant evidence” from previous plots in Britain that involvement in conflict overseas can prompt extremists to engage in terrorism here.

It points that 41 per cent of the 37 terrorists involved in eight major bomb plots in this country between 1999 and 2010, including the July 7 London bombings, had foreign fighting experience and warns that Britons who have fought overseas are “disproportionately involved in some of the most serious offences” carried out here.

The report states: “Previous terrorist behaviour in the UK shows that the threat from returning fighters and the ‘blowback’ from foreign jihadist conflicts should not be underestimated. Allowing for deaths in the battlefield, a conservative estimate would suggest that approximately 40 returning fighters will seek to re-engage in terrorist activity.”

Hannah Stuart, the report’s author, added: “All the evidence suggests that some of those who come back will try to mount attacks in this country.”

The report says that measures such as “selective use of the treason law” and the suspension of benefit payments could be used to deter people from travelling to fight for the Islamic State or other terror groups.

The findings follow a warning from the Met Commissioner that 500 to 600 British residents have so far taken part in the Syria and Iraq conflict and that close to 200 have already returned to the capital.

The official terrorism threat level has also been raised to “severe” – meaning that an attack is highly likely – in response to growing concern within the intelligence agencies and police about the danger faced by this country.