BBC Radio 1's website pulled in more than 3m users while Radio 4 contributed around 1.5m. The month beat the previous record of 8.1m, set in November 2005.
BBC Radio Five Live showed the biggest increases, with the main site tipping the million mark, helped along by host Jeff Randall's lengthy chat with media tycoon Rupert Murdoch. Offshoot sites 606 and Sportdaq also showed huge growth.
Drama on BBC Radio 4 continued to attract a massive audience with more than 1m on-demand requests. 'The Archers' remained the most requested programme while Radio 4's Afternoon Play saw more than a quarter of a million people tuning in through the site.
With the BBC expanding its audio podcast trial, the site also generated a record 1.9m MP3 downloads.
In all, more than 17m hours of live and on-demand radio were consumed through the BBC's radio websites last month.
The most viewed videoclip on the website was a spoof fitness video featuring radio DJs Scott Mills, Chris Moyles and Colin Murray.
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