A teenager from Essex has been sentenced to at least six years in prison after admitting he was planning a terror attack on the police and military.

Matthew King, 19, from Wickford, was arrested by the Metropolitan Police on 18 May last year, and charged with the preparation of terrorist acts.

King had told an online girlfriend about his desire to torture and kill an American or British soldier and had also posted an image of a police officer with the caption: “Target acquired.”

He had been scoping out targets and had previously talked about wanting to carry out a “mass casualty attack” on the public.

Detectives believe King had selected his target and was preparing to carry out an attack when they arrested him.

Police were alerted by calls to the Anti-Terrorist Hotline and local police from members of the public, telling them that King was becoming “increasingly extreme and unstable”.

King’s mother also reported him to the Prevent counter-terrorism programme.

He was given a life sentence with a minimum tariff of six years in prison.

Judge Mark Lucraft KC praised King’s mother, saying: “She took the very bold step of alerting Prevent when she had concerns for her son.

“That cannot have been an easy thing to do in the first place and in my view she absolutely the right thing.”

Calls were made to police between 14 and 18 April last year, after King shared a video on a WhatsApp group, according to prosecutors.

The video shared on 13 April featured a still image of a man holding a knife and the words on the screen: “Those who said that there is no jihad and no battle. They are lying! Our jihad will continue until the Day of Judgment!

“Now the battle has begun. So take out your sword, O youth, and destroy the kufr [infidels].”

Commander Dominic Murphy, head of Scotland Yard’s Counter-Terrorism Command, said he believed an attack was “imminent” when they arrested King, four weeks later.

A written plea document from the defence said King “actively desired” to travel to IS-controlled areas of Syria to join their activities, seeking advice on how to get there and using a WhatsApp group to discuss plans.

He had also made videos expressing his admiration for the terror group and said if his plan to go to Syria was disrupted, he would carry out an attack in the UK.

King also set up an account with the online store Knife Warehouse in an effort to buy a blade in December 2021, and in March 2022, bought “tactical” gloves and goggles from an army surplus store.

He had been spending a great deal of time online, during the COVID-19 lockdown, using his phone to view a large number of extremist videos, including IS propaganda showing suicide bombers and mass executions.

In conversations on Snapchat with a female friend, later recovered from his phone, King discussed his desire to travel to Syria to take part in violent jihad and talked about becoming a martyr, on a number of occasions.

King told the girl that he was training for jihad and wanted to kill people. In one particularly graphic exchange, he talked about his desire to torture and kill an American or British soldier.

He also joined an online chat group, in which he discussed subjects including terrorist financing and different routes to travel out to Syria.

At one point, he updated his WhatsApp status to “Kill non-Muslims, wherever you see them”.

On two occasions between March and May 2022, King took a photograph of police officers in Stratford, east London, and on a third occasion, filmed the side entrance of Stratford Police Station as he walked past.

Last May he also made a short film of an army barracks in Stratford.

Speaking on King’s behalf at the Old Bailey, the defence said King was “immature” and the likelihood of an act of terrorism was “remote”.

His barrister Hossein Zahir KC argued that despite such incidents of “offensive and abusive” behaviour, King was “slowly and steadily” disengaging from the excesses of extremism.

In a prison phone call, King told his mother: “I’m not extreme anymore.”

But police said King had a “firm intention and desire to carry out a terrorist attack” and had discussed committing indiscriminate attacks on the public, as well as showing a “sinister interest” in police officers and army personnel.

Mr Murphy described him as a “very committed individual” and said officers were struck by how quickly he went from developing extremist views to being almost ready to launch a terrorist attack.

Despite pleading guilty to preparing acts of terrorism, King was “somebody who doesn’t demonstrate any kind of remorse or regret for their behaviour or their activity”, Mr Murphy added.