The terrorist threat to the UK is rapidly evolving and placing huge demands on the shoulders of junior officers who have to make life-and-death decisions in split seconds, according to a former senior officer with Scotland Yard.

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Scott Wilson, who rose through the ranks of SO15, the police counter-terrorism division, and became the National Coordinator for Protect and Prepare, the UK’s national counter-terrorism policing initiative, said the threat involved more individuals who were often vulnerable and volatile and who had only tenuous links to terrorist organisations.

Last Sunday Sudesh Amman was shot dead by police after stabbing two people in a terrorist attack in Streatham, London. The 20-year-old, who had just come out of prison for terrorism offences, had been radicalised, at least in part, by interacting with others via the internet.

“Fifteen or 20 years ago, if people wanted to become a terrorist they had to be part of an organisation,” Wilson told the Observer. “Now people get radicalised online by watching videos or listening to sermons.”With the terrorist threat level currently at “substantial”, police forces regularly conduct counter-terrorism exercises to formulate new ways of tackling emerging threats. The last major exercise – codenamed Strong Tower – involved 1,000 police officers at locations across London. But such exercises are a strain on police resources, Wilson acknowledged.

“There’s no point exercising when you’ve got suspects on the street who need surveillance. So you need to get the balance between exercises and live investigations.”

Scotland Yard assistant commissioner Neil Basu, who has national responsibility for counter-terrorism, said last year that Britain’s police forces were conducting around 800 live terrorist investigations.

Wilson said that to place one individual under surveillance would involve between 15 and 20 officers. “In terms of counter-terrorism, you have the ongoing investigations, then there’s the people being released from prison, and then there’s the likelihood of returnees from Syria and Iraq.

“You add those three together and over the next few years it’s going to be a very busy time for CT policing,” he said.

It is estimated that around 150 terrorists will be released from prison in the next three years.

Since the Paris attacks in 2015 the government has invested £150m in training more firearms officers. While this has seen more armed officers in plain clothes on the streets, it has also seen a shift in responsibility.

“It’s the officers in the street who are going to have to respond,” said Wilson, who is now a director of a security management consultancy, Crisis Management Solutions.

“They have to make the dynamic decisions about whether to carry out the arrest or, as we’ve seen on the last few occasions, have had no option but to use lethal force. They have to make the decision based on what’s in front of them.

“They will be in contact with control rooms, but it’s happening too quick. They are not asking for authority, they’re making decisions up off their own bat. Although they are very experienced officers, they are usually junior. It’s PCs and sergeants who are having to take that responsibility.”