Metropolitan police chief Sir Bernard Hogan-Howe has said he believes a terror attack in the UK is "a case of when not if".
Because Britain's terror threat level continues to be set at a "severe" level, "it means an attack is highly likely," Hogan-Howe wrote in a blog post on the London police service's website. "You could say it is a case of when not if."
Following several terrorist attacks across Europe over the last year, most recently in Nice, France, where 84 people were killed when Mohamed Lahouaiej Bouhlel drove a truck through crowds gathered on the city's sea-front, Hogan-Howe recognised that people were looking for reassurance that similar incidents would not occur in the UK.
But, he added that even as the police officer in charge of preventing such an attack, "I am afraid I cannot do that entirely".
Instead, he said, he sought to offer reassurance about the ways in which police have continued to prevent such attacks since the brutal, al-Qaida-inspired murder of Lee Rigby in 2013.
"I can point to the plots we have foiled, including terrorists who we have put behind bars after we foiled their advanced plans to murder police officers leaving Shepherd's Bush police station to walk their beats," Hogan-Howe wrote.
"Another saw a British based terrorist with advanced plans to carry out a Lee Rigby style attack on US soldiers, using a car and knives to murder the servicemen and potentially detonate a bomb as part of the murderous plot. He was thwarted thanks to joint work by the police and MI5."
He praised strong communications between intelligence agencies and police teams nationally, as well as the UK's strong gun control laws in halting attacks on Britain.
"In short we have a professional, joined up and national network of intelligence-led investigations," he said.
Hogan-Howe also highlighted a strong culture of tolerance and acceptance as a key strength in the police's fight against terrorism.
"Our approach to Muslims is no different because these attacks purport to be committed in the name of Islam," he said.
"We don't stigmatise the millions of British Muslims whose values and faith completely reject the terrorists' litany of hate. We are a nation that accepts difference but builds communities with shared values.
"We should be proud of that - it is another reason why the UK is a hostile environment for those intent on murderous terrorism.
"It means that the vigilance of our officers is backed-up by millions more eyes and ears. These are whole communities united in defeating terrorism."
Hogan-Howe encouraged people to continue to live their lives as normal, because "defeating this terrorism is as much about refusing to be afraid as anything else".
"We should be positive and enjoy our freedoms," he added.