Circulation for the Sunday title, owned by Guardian News & Media, fell 18.06% from 542,075 in January 2006 to 444,186 this year, according to the Audit Bureau of Circulations.
A spokeswoman for The Observer said the circulation was particularly high in January 2006 because of the change of format, the launch of Observer Woman and heavy promotion. The title's circulation fell 10.65% month on month immediately after the relaunch to 484,357 in February 2006.
This month's figure is closer to The Observer's pre-Berliner circulation level, which was 430,401 in December 2005.
Meanwhile, average circulation over the six months of August 2006 to January 2007 fell 0.25% compared with the same period the previous year.
Anthony Gibson-Watt, media director at Zed Media, said this did not mean the investment in the Berliner presses was wasted. "Agencies and advertisers welcome the change," he said.
The Independent on Sunday was the only title in the Sunday quality market to have a year-on-year circulation increase (2.66%) this month, helped by a CD giveaway and a rise of about 10,000 in combined bulk and international sales.
The Sunday Telegraph fell by 3.35% year on year, while The Sunday Times fell 5.06%.
PROMOTIONS
- Evening Standard Gave away free mobile phone chargers
- The Daily Telegraph Gave away a Beatrix Potter audio CD, a puzzle special, and ran an 'eat out for £5' promotion
- The Guardian Gave away posters, stickers, a guide to free events, and a DVD of What's Eating Gilbert Grape
- The Times Gave away a book of name meanings, a Great Gastro Pub Guide, and a Michael Palin DVD. Also ran an "eat out for £10" promotion and a promotion in WHSmith for £1 off magazines with The Times
- The Independent Ran a Test Your Brain CD, bird song CDs and bird post cards
- Daily Mirror Gave away Merlin Premier League albums and a National Geographic wildlife DVD, stickers and posters.