"Our threat level has been at "severe" for two years - it remains there. It means an attack is highly likely - you could say it is a case of when not if," said the Metropolitan police commissioner Sir Bernard Hogan-Howe.
After recent major attacks, he said: "I feel and understand that fear and as the police officer in charge of preventing such an attack know that you want me to reassure you. I am afraid I cannot do that entirely."
"Indeed, there are lots of things working in our favour. The relationship between MI5, MI6 and the police is a world-beater. It has given us an advantage in intelligence and ultimately foiling plots. My team at Scotland Yard co-ordinate the national police effort across the country, working with specialist hubs in other regions. In short we have a professional, joined up and national network of intelligence-led investigations."
"Our gun controls and the simple fact we are an island mean that terrorists in the UK would struggle to get the firearms required to repeat attacks similar to those we have seen on the Continent. This gives a solid base as it means the UK environment is immediately hostile to the terrorist."
He credited the UK's tolerance and acceptance meant the country doesn't "stigmatise the millions of British Muslims whose values and faith completely reject the terrorists' litany of hate".
The number of armed police ready to respond to such an attack has increased from 600 to 2,800 since last year's attacks in Paris, said Hogan-Howe, while specialist counter-terrorism officers are now on-hand 24 hours a day.
"I realise that some of what I am telling you today is not reassuring. I hope that some of it is more so. The threat we all face is very real, no-one watching events in Europe can think otherwise, but it is important that we have a shared understanding of the work that goes on every day to stop attacks happening and to prepare for the time when we are faced with this terrifying threat."