Britain's GCHQ was responsible for an attack on Belgian state-owned telecoms company Belgacom in an operation dubbed "Operation Socialist", according to documents released by whistleblower Edward Snowden.
The documents - copies of a "top secret" presentation at GCHQ in Cheltenham - indicate that GCHQ has had access to parts of Belgacom's network since 2010, via a subsidiary called Bics.
Belgacom customers include all the main institutions of the European Union, including the European Commission, the European Council and the European Parliament, and any information arising from the operation will almost certainly have been shared with the US, too.
The documents have been released to journalists working for Der Spiegel newspaper in Germany, as the UK's national press has reportedly been gagged by government D-Notices - official orders to cease publishing about specific matters of national security.
"According to the slides in the GCHQ presentation, the attack was directed at several Belgacom employees and involved the planting of a highly developed attack technology referred to as a 'Quantum Insert'," claims the Der Spiegel report.
It continues: "It appears to be a method with which the person being targeted, without their knowledge, is redirected to websites that then plant malware on their computers that can then manipulate them. Some of the employees whose computers were infiltrated had 'good access' to important parts of Belgacom's infrastructure, and this seemed to please the British spies, according to the slides."
Perhaps more alarmingly for Belgacom, the slides suggest that the spies at GCHQ were close to cracking the security for the company's central roaming router used to process international mobile telecoms traffic.
The presentation indicates that GCHQ intended to use this access to execute "man in the middle" attacks on targeted smartphone users.