The site, which broke the 25 million barrier in October, posted a slight gain in unique users of 0.26 per cent to 26,043,707.
In second place was Telegraph.co.uk with 22,958,505 unique users - a decrease of 1.08 per cent (250,397) from October's figures.
Mail Online also recorded a slight drop in unique users and was replaced as the UK's third most popular newspaper site by Times Online, the site which saw the largest month-on-month gain in unique users (up 5.49 per cent).
The full figures for the six audited titles and Mirror Group's websites are listed below - the percentage indicates the month-on-month change compared with October's ABCe report:
Guardian.co.uk
Unique users: 26,043,707 (+0.26 per cent)
Mail Online
Unique users: 20,869,627 (-1.54 per cent)
Telegraph.co.uk
Unique users: 22,958,505 (-1.08 per cent)
Times Online
Unique users: 21,598,720 (+5.49 per cent)
Sun.co.uk
Unique users: 16,427,790 (-4.09 per cent)
Independent.co.uk
Unique users: 8,888,221 (+5.06 per cent)
Mirror Group
Unique users: 5,916,155 (+1 per cent)
