Encrypted communication services are eroding the ability of law enforcement to detect dangerous offenders, an official report has warned.
The National Crime Agency (NCA) revealed that its officers observe the use of encryption in criminal contacts across all threat areas it tackles.
Publishing its strategic assessment of serious and organised crime for 2018, the NCA highlighted the impact of technological developments on authorities' ability to collect evidence and intelligence.
Since 2010, communication providers have moved towards offering encrypted services "by default", the report noted.
The majority of internet traffic is encrypted while publicly-available mobile apps offer end-to-end encryption as standard - meaning messages are encoded so that only the sending and receiving devices can read them.
The NCA's assessment acknowledged that encryption provides important benefits to the public and economy, enabling digital commerce, ensuring security on the web and increasing privacy.
"However, such technology has become an enabler to criminality, presenting serious challenges for law enforcement," it warned.
"While encrypted communications platforms are legitimate products welcomed by consumers and privacy advocates, they will increasingly erode law enforcement's capability to detect and deter criminal activity."
The issue was also flagged up by the agency's director general, Lynne Owens.
She said: "This year's assessment shows that organised crime groups are exploiting digital technology, for instance using encryption to communicate, and dark web market places to aid their activities."
The 58-page review summarised the threat picture in child sexual exploitation and abuse, modern slavery and human trafficking, organised immigration crime, cyber crime, money laundering, drugs and guns.
It said serious and organised crime affects more UK citizens more often than any other national security risk - and warned the threat is increasing in volume and complexity.
In detailed findings, the wide-ranging assessment said:
-Analysis known as "mapping" indicates there were 4,629 organised crime groups in the UK at the end of last year.
-The UK's 3,000-plus airstrips present opportunities for criminals to exploit the border to facilitate the illegal entry of migrants.
-The closure of migrant camps at Calais and Dunkirk has reduced the scope for "opportunistic" attempts to enter the UK from France, but Belgium has become a location of "greater focus" for people smuggling gangs.
-There has been an upward trend in criminal firearms discharges, with the majority of weapons not having been previously used - indicating a "fluid illicit supply" via UK and overseas sources.
-Offending by under-18s forms a rising proportion of reported child sexual exploitation and abuse.
-The UK is a "prime destination" for corrupt foreign "politically exposed persons" to launder the proceeds of corruption.
-In cyber crime, the distinction between nation states and criminal groups is becoming more blurred - making attribution of attacks increasingly difficult.
-Mobile phones in prisons give offenders involved in serious organised crime the means to play a "full role" in major criminal enterprises "virtually unaffected by physical confinement".
-The scale of modern slavery and human trafficking in the UK is continually and gradually increasing.
On Brexit, the NCA noted that the result of the referendum has had little impact on criminal activity or international law enforcement co-operation to date.
It is anticipated that many criminals will strive to take advantage of opportunities that Brexit might present, according to the report.
It cited as possible examples the design and implementation of a new customs system or increased challenges in locating and extraditing international fugitives if the UK were to lose enforcement or intelligence sharing tools.
However, the report added that some of the effects of Brexit have the potential to work in favour of law enforcement, including greater discretion over the movement of goods and people.