Taliban fighters targeting British troops in Afghanistan are using Skype voice-over-IP phones to evade detection.
Security sources have told the Evening Standard that unlike traditional mobile calls, which can be monitored by RAF Nimrod spy planes, Skype calls are heavily encrypted.
Taliban leaders had previously been known to use satellite phones, which could be tracked and located by western forces.
The British and American governments are said to be investing resources to crack voice-over-IP (VoIP) codes.
"The trouble with this technology is that it is easily available but devilishly hard to crack," a security source told the Standard. "The technology can now be accessed on mobile internet devices and the country's mobile phone network is expanding rapidly."
Skype is owned by eBay and has around 300m user accounts worldwide.
Sir David Pepper, head of government listening centre GCHQ, has previously complained that internet calls are "seriously undermining" his organisation's ability to intercept communications.
Read more about VoIP encryption:
Fast Guide: VoIP encryption>>
Top 10 VoIP tips>>