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A London teenager who flew abroad to fight alongside Islamist extremists in Syria has escaped prosecution after a landmark ruling by the Attorney General.

The 17-year-old boy set off after leaving letters at his family home saying that he intended to wage violent jihad to help set up an Islamic state. He reached Turkey before changing his mind and returning to Stansted, where he was arrested by Scotland Yard counter-terrorism officers.

Prosecutors decided to charge the teenager with preparing acts of terrorism after he refused to admit his guilt and take part in a mandatory counter-radicalisation programme.

But the prosecution had to be abandoned after the Attorney General, the Government’s top law officer, refused to authorise the charge. He ruled that taking the boy to court would not be in the public interest because of his age and immaturity and the fact that he came home before entering Syria to fight.

The decision is the only occasion on which the prosecution of a Syria-related offence has been vetoed on the grounds of public interest despite the existence of enough evidence to justify charges.

Details of the case — uncovered by the Evening Standard — came as Prime Minister David Cameron announced that a new counter-extremism Bill will include powers to ban those who use “hate speeches” to undermine democracy and British values.

The legislation, to be unveiled in the Queen’s Speech later this month, will also create new extremism disruption orders. They will permit the closure of mosques or other venues where radicalisation takes place. The broadcast regulator Ofcom will also be given stronger powers to stop the transmission of extremist material.

Despite the crackdown — which Home Secretary Theresa May today said was aimed at those wanting to divide Britain into a “them and us” society” — news of the decision not to prosecute the London teenager will be welcomed by those who have called for a more flexible approach by the authorities to returning extremists. It follows warnings by some security experts that an excessively hardline approach could backfire and fuel radicalisation.

Details of the teenager’s case, which the Evening Standard first learned of last year, have been kept secret until now, but were finally confirmed today in a statement from the Crown Prosecution Service.

It said that the teenager was reported missing in January last year after a note was found at his home explaining that he had gone to Syria to take part in “jihad”. Another letter was also found that “included evidence suggesting... a desire to engage in violence, to establish an Islamic state” and that he “understood the legal consequences of such activity”.

The CPS said that, on the day he was reported missing, the boy had also made contact from Turkey to tell a relative that he had “made a mistake and wanted to return home to the UK”.

A family member went to fetch him and he was arrested on suspicion of preparing acts of terrorism.

Prosecutors initially offered him the chance to avoid court by accepting a conditional caution. That would have required him to admit his guilt, but left him facing no further sanction other than participation in the Government’s “Channel” counter-radicalisation scheme.

The boy, who is described as being of previous good character and from a “stable family background”, refused. That prompted prosecutors to decide to charge him instead.

But they were over-ruled by the Attorney General, who in law must sanction the prosecution of offences “connected with the affairs of another country”.

He decided that the teenager’s age and change of mind while in Turkey justified a more lenient approach.

A CPS spokesman: “Being unable to offer a conditional caution, we considered that a prosecution would then be required in the public interest. On this occasion, the Attorney General declined to give consent on public interest grounds. This is the only occasion when consent has not been granted for such offences.”

The Attorney General’s office said the decision to veto prosecution had been taken by his deputy, the former Solicitor General Oliver Heald. Another 29 defendants are awaiting trial on terrorism charges connected to Syria.