The website recorded a 22.4% leap in traffic during September, recording its highest ever unique user figure of 12,536,438.
The website credited its growth to its coverage of the Madeline McCann saga, the Rugby World Cup, Formula 1 season. In addition growth was credited to improvements in search engine optimisation.
In comparison, the Mail Online, previously the second largest site, slipped to third place drawing 11.7m unique users for the month.
Sun Online posted 10.7m unique users in the month, but page impressions fell by 23m to 216m.
Hot on its heels was the telegraph.co.uk, which has broken through the 10m barrier for the first time, drawing 10,645,220 unique users.
But Guardian Unlimited is still leading the way across the board with more than 16.7m users last month – an increase of nearly 1m. It also registered 158m page impressions – up from 155m in August.