In the wide-ranging speech, Mr McCallum talked about MI5 operating in “a new era” with terrorism remaining an “ever-present threat”.

He revealed MI5 and police had disrupted 19 late-stage attack plots and intervened in “many hundreds of developing threats” since the start of 2020.

The “aggregate scale of the terrorist threat remains huge”, he said, explaining how his teams were mostly focused on individuals or small groups, rather than larger established networks.

One in five of the 232 terrorism arrests last year were of children under 17, he said.

He also said al-Qaeda and Islamic State group were “once again becoming more ambitious” and “taking advantage of instability overseas to gain firmer footholds”.

Speaking about threats from state actors including China, Russian and Iran, the director-general said as well as methods of espionage, state actors are “descending into ugly methods MI5 is more used to seeing in our terrorism casework”.

State threats include espionage against the UK’s Parliament, universities and critical infrastructure.

He warned that would-be “proxy” actors are viewed by Russia as disposable, saying “when you’re caught, you’ll be abandoned”.

While on Iran, he also said MI5 had tracked more than 20 potentially lethal Iran-backed plots in the last 12 months.

Meanwhile, referring to a court battle with the BBC about false evidence given by the agency to three courts, Mr McCallum said MI5 had been “justly criticised” by the courts.

“I want to affirm, on record, the importance MI5 attaches to accountability and oversight,” he said, adding: “Part of why MI5 exists is to defend our national way of life – a way of life which includes the checks and balances of rigorous independent oversight”.

The director-general also spoke about Artificial Intelligence (AI), saying while the technology brings immense benefits, it would be reckless to ignore the potential for it to cause harm.

McCallum ended the speech by paying tribute to the “quiet service” of “talented, selfless, often courageous MI5 people” and the work they do behind the scenes.