The EU has reported a "serious" cyber attack on the Commission and External Action Service on the eve of a summit in Brussels, a spokesman told the BBC.
Crucial decisions on the future structure of the EU, economic strategy and the ongoing war in Libya are to be discussed at the two-day talks.
Details were not given but other sources compared the attack to a recent assault on France's finance ministry.
"We're often hit by cyber attacks but this is a big one," one source said.
The European Commission has been assessing the scale of the current threat and, in order to prevent the "disclosure of unauthorised information", has shut down external access to e-mail and the institutions' intranet.
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