Encryption "frustrates" police investigations on a daily basis, Britain's most senior officer has revealed, as she warned the terror threat has shifted in tempo this year.
Cressida Dick said there are large numbers of "apparently volatile" individuals in the UK who may have been inspired largely through the internet.
In her first major speech on counter-terrorism, the Metropolitan Police Commissioner also disclosed that six plots have been foiled in the last four months.
She described the "pursue" element of the national counter-terrorism strategy, which aims to stop attacks, as "formidable".
"But the challenges are great," Ms Dick said.
"Increasingly, encryption frustrates our investigations every day.
"We have had unprecedented numbers of UK citizens travelling to these conflicts in largely ungoverned spaces.
"Progress on the ground in Syria and Iraq does not necessarily translate into a reduction in threat here.
"And we have large numbers of apparently volatile individuals in the UK, some of whom become determined to die, who may have been inspired largely through the web and decided on methodology learned from there too."
The Scotland Yard chief's remarks on encryption underline the challenge for the Government as it seeks to clamp down on online "safe spaces" where terrorists and other serious criminals can communicate without detection.
Scrutiny has focused on so-called end-to-end encryption, which is built in to messaging services such as WhatsApp and means that messages are encoded in such a way that only the sending and receiving devices can read them.
Ms Dick said the modern threat, more than ever, "includes the encouraging of others to commit atrocious acts".
She continued: "That virus can infect communities and is spreading faster and more easily due to the internet. We need to get explicit content taken down as quickly as possible."
In her speech at Mansion House in the City of London on Thursday night, she also:
:: Predicted the number of existing and former subjects of interest for police and MI5 will rise from the current total of 23,000;
:: Warned that Daesh - also known as Islamic State - and al Qaida are exploiting technology and the "relative ease of international travel" to promote their ideology and project their threat across borders;
:: Flagged up the threat from the extreme right-wing and those motivated by racist hatred.
Britain has been hit by four attacks this year and Ms Dick said "the tempo has changed" since March.
"What we are seeing is now being described by the experts as a 'shift' in threat, not a spike," she said.
In addition to the attacks at Westminster, Manchester, London Bridge and Finsbury Park, Ms Dick said six "attack planning plots" have been thwarted in the last four months.
"We can expect that figure to rise," she added.
Police and MI5 are running more than 500 investigations into 3,000 individuals assessed as posing the greatest threat.
There are a further 20,000 former subjects of interest whose risk remains subject to review.
"I anticipate these numbers will grow," the commissioner said.
She described the task of working out who poses what risk at any one time as "really hard", raising the prospect of authorities having to intervene earlier in investigations.
"The degree of risk, volatility and uncertainty means we don't have the luxury of long, careful intelligence and evidence-gathering operations to build a clear picture," Ms Dick said.
"This will mean we sometimes don't have the evidence needed to charge."