The News Group Newspaper-owned tabloid dropped below the 10-million unique user barrier in October, recording 9.2 million users compared to 10.7 million during the previous month. The Sun now has fewer than half the unique users recorded by Guardian Unlimited.
The October ABCe figures placed The Sun in fifth place among the newspapers that make website statistics available to the public.
Telegraph.co.uk overtook The Sun to land fourth place with 11.1 million unique users in October. But it was Guardian Unlimited and Mail Online were the most successful. Guardian Unlimited achieved a record 18.4 million unique users in that month, consolidating its position as the most visited UK newspaper online.
Industry watchers expect Guardian Unlimited to build on last month's increase of almost two million unique users when its Guardian America online venture kicks in.
Guardianamerica.com launched on 24 October to cater for its large US audience. More than 60% of Guardian Unlimited's online readership is based overseas, although the Daily Mail's website has the highest percentage of foreign-based readers, with only 28% of unique users accessing it from inside the UK.
Meanwhile, the Mail's site continued its rise up the rankings in October to become the second most read UK newspaper website, according to the ABCe figures.
Dailymail.co.uk recorded 13.5 million unique users, up from 11.7 million in September.