BBC Global News director Peter Horrocks said it appeared to be "deliberate censorship", adding that the BBC was complaining to the authorities.
Other major news websites were functioning as usual, and the reason for the blackout remained unclear.
Since the 2008 Beijing Olympics, the BBC's English-language site has been generally available.
However, other BBC services including its Chinese-language news website has been blocked largely since it was launched.
BBC World TV - as with all overseas broadcasters - suffers intermittent blackouts particularly when reporting China stories.
Mr Horrocks said: "The BBC provides impartial, trusted news to millions of people around the world, and attempts to censor our news services show just how important it is to get our accurate information to them."
The last time the BBC's English-language website was disrupted was during the corporation's coverage of activist Chen Guangcheng's escape from house arrest in April 2012.
Access to the BBC's English-language website has been blocked across China, the corporation has said.