The data follows a series of NRS PADD reports last year, which were based on statistics from the NRS and online results from UKOM/Nielsen.
As of January, the way the data has been collected has been changed, with the report using comScore instead "as its supplier of website audience estimates".
In a statement NRS said "there are often considerable differences between the website estimates produced by comScore and Nielsen, which make it inappropriate to compare the two."
The data released today, therefore, which looks at NRS data covering January to December, and comScore data for November, is to represent a "2012 comparison point for 2013 subsequent quarterly releases and beyond".
Today's results place the Daily Mail top of the tables across national titles for monthly combined readership, with a print and website readership of just over 20 million. This includes a website only readership of 8.7 million.
In comparison the Sun follows in second with a combined monthly readership of 17.4 million.
For weekly and daily combined readership the Sun takes back the top spot, with 13 million and 7.4 million respectively, compared to 11.3 million and 6.7 million for the Mail.
Within 'quality' titles only the Guardian secured the highest combined monthly readership with 12.7 million, followed by the Telegraph with almost 12 million.
When taking into account the Observer's readership also the Guardian's total readership rises to more than 13 million a month.
In a statement chief commercial officer for Guardian News & Media David Pemsel said the results "reaffirms our position as Britain’s most successful quality newspaper brand with over 5.4 million people in Britain reading our content in print and online every week, and over 13 million every month".
"We are seeing record digital traffic across our online platforms and our journalism is being read by more people than ever before. This cross-platform success – both in Britain and globally – enables us to offer advertisers increasingly innovative commercial solutions and we’re constantly exploring the best possible advertising opportunities for our clients.