British intelligence officers based at the Government Communications Headquarters (GCHQ), the state eavesdropping service, attacked the 67-page magazine, leaving most of it garbled, British newspapers said.
Instead of being able to read how to "Make a Bomb in the Kitchen of Your Mom," readers were greeted with computer code which actually contained recipes from The Best Cupcakes in America, published by U.S. chat show host Ellen DeGeneres.
The Washington Post reported that the British action followed a dispute between the CIA and the newly formed U.S. Cyber Command.
The cyber unit had wanted to block the al Qaeda magazine but the CIA, which had countered such an attack would expose sources and intelligence methods, won the debate and declined to allow an attack on Inspire.
Last year, Britain's new National Security Strategy placed cyber attacks as one of the top threats to the country, and ministers have repeatedly spoken out about the danger posed by extremist Islamist websites.