Ad spend in the 17 weeks to 29 April fell 11% and total revenue fell 4%.
Circulation revenue fell 4% in March and April as it lost some of the ground gained from the closure of the News of the World last year.
Total circulation revenue for the four-month period, however, increased 1%.
The group also expects to see a fall in ad revenue in May.
The Sunday Mirror, now the largest Sunday red top, recorded a 9.06% fall month-on-month to 1,594,293 in February following the launch of the Sunday Sun, according to the latest ABC audit figures.
Former chief executive of the group, Sly Bailey, stepped down last week and the publisher is yet to name a replacement.
The group says its recently launched daily deals site Happli is “progressing well” and is likely to roll out to further cities before the end of the year. It is currently live in eight UK locations and counts 100,000 subscribers.
In a trading statement, the group says it expects conditions to “remain challenging” for the rest of the year with continued “volatility” in revenue trends.