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.
Staff have been asked to change their passwords.
'Serious cyber attack'
"The Commission and External Advisory Service are subject to a serious cyber attack," Antony Gravili, the spokesman for the inter-institutional relations and administration commissioner, told the BBC News website.
"We are already taking urgent measures to tackle this. An inquiry's been launched. This isn't unusual as the commission is frequently targeted."
Mr Gravili added that he had no information the attack had been linked to the EU summit.
France's finance ministry came under a cyber attack in December that targeted files on the G20 summit held in Paris last month.
Confirming the attack, Budget Minister Francois Baroin said an investigation had been launched.
Paris Match magazine said a sustained cyber attack had sought documents related to the G20 and international economic affairs.
More than 150 of the French ministry's 170,000 computers were affected.