London Assistant Commissioner Mark Rowley said that since former US spy contractor Edward Snowden revealed the extent to which intelligence agencies could intercept online traffic, new technologies had emerged that made plots harder to foil.
"We have a growing Achilles heel that, if it is not tackled, will slowly diminish our ability to keep the public safe," Rowley told the Royal United Services Institute, a London think-tank.
"If we are glibly creating a safe operating environment for criminals and terrorists, we are going to regret it."
His comments on social media and the impact of Snowden's revelations in 2013 echoed similar expressions of exasperation from Prime Minister David Cameron and the head of Britain's domestic spy agency, MI5.
Cameron's government is planning new laws, due to be detailed later this month, to bolster the surveillance capabilities of spies and police. They will face stiff opposition from privacy and human rights campaigners who say they represent an assault on freedoms.
Britain is on its second-highest alert level of "severe", meaning a militant attack is considered highly likely, mainly due to the threat the authorities say is posed by Islamic State and Britons who have gone to join the fighting in Syria, a number which Rowley said had now reached 750.
British police arrested a record number of people last year on suspicion of terrorism offences, and Rowley said there were more than 600 investigations ongoing across the country.
He added about 600 people were being referred to counter-terrorism police every month because of worries they were being radicalised, although he said about two-thirds of these were later cleared of any terrorism link.
Against this backdrop, Rowley said the police's greatest concerns were the growing "no go" areas online and the development of encryption that allowed terrorists and criminals to communicate under the radar.
"All counter-terrorism operations are now more patchy in their intelligence than we've ever seen before," he said.
"We have had occasions where we have had to prolong dangerous operations and to delay arrests until we can confidently obtain evidence because encryption has been slowing our progress down. That delay makes us all very nervous."
He said while some social media firms were helpful, others, who he declined to name but indicated were numerous, refused to assist inquiries and designed products they knew would hinder law enforcement agencies.
Others would only pass on information to police on the condition the person involved was informed.
© Thomson Reuters 2015
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