The Federal Bureau of Investigation warned Barack Obama's and John McCain's campaign teams that their computer networks had been hacked into by a foreign organisation during the presidential contest.

The hacking was serious and sustained and saw a significant amount of information downloaded from the campaign network, which the security agencies believed was sought by a foreign entity seeking to know more about the contenders' policy positions.

Details of the incident were revealed by a Newsweek reporting team that enjoyed behindthe-scenes access to both campaigns on condition that nothing was published until after polling day.

Federal Bureau of Investigation and Secret Service agents informed the Obama team of the cyberattack this summer. The McCain campaign was similarly attacked.

"You have been a problem way bigger than what you understand," the agents reportedly warned the Obama campaign. "You have been compromised and a serious amount of files have been loaded off your system."

Newsweek said the problem prompted Josh Bolten, White House chief of staff, to call David Plouffe, Mr Obama's campaign manager, and say: "You have a real problem . . . you have to deal with it."

Dana Perino, White House press secretary, declined to comment.

Richard Kolko, FBI spokesman, and Russ Knocke, spokesman for the homeland security department, said they had no information on the attack.

Spokespeople for both campaigns did not respond to questions.

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