Mirror Online's total monthly user numbers fell from 19,092,360 to 13,117,464 between January and February, a fall of 31.29%, according to the latest ABC figures.
The site relaunched on February 8, offering greater compatibility with Facebook and a complete redesign.
But a source close to Mirror Online played down the fall, saying that it was a result of the "rewiring" of millions of URLs, which impacted its traffic through Google search. The source said traffic would likely return in the coming months.
Publisher Trinity Mirror is gearing up for a raft of paid-for digital newspaper e-editions and mobile devices.
The MailOnline juggernaut temporarily stalled, failing to hit the 100,000,000 unique browsers that it looked set to hit in February.
MailOnline attracted 91,148,015 unique browsers in February according to ABC, down 8.13% on the previous month, although up 78.97% on the year.
All the major national newspaper websites were down month on month, except for The Sun Online, which could be attributed to February being shorter month.
But the MailOnline said today (22 March) that it had overtaken The New York Times' daily traffic for the first time, according to comScore.
MailOnline previously overtook the NYT's monthly traffic in December to become the world's most popular newspaper website.
Across the rest of the market, guardian.co.uk did not report an ABC figure in February.
Its January figure, when it last reported, was 63,171,662 unique browsers, retaining second place among newspaper websites, according to ABC.
Telegraph.co.uk is still third with 48,292,476 unique browsers in the month.
Independent.co.uk reported 13,126,707 unique browsers, down just 2.34%.
Follow John Reynolds on Twitter @johnreynolds10
|Multi-platform ABC report February|
|February||Month on month||Year on year|
|Daily average (global)||Total monthly (UK)||daily change (%)||daily change (%)|
|Mirror Group Digital||662,162||7,015,427||-25.01||20.77|