The Guardian's sales estimate for the first week contrast with estimates for the second week of the Berliner, from The Times. It said the increase had dropped to around 20%.
Along with The Times, The Daily Telegraph claimed its sales had not been affected by the arrival of the Berliner.
However, no estimates were available for The Independent's sales, which stand to suffer most from a revitalised Guardian.
Andrew Mullins, general manager at Times Newspapers Limited, said Berliner Guardian's launch week coincided with The Ashes victory and that most newspapers had increased sales during that period.
He added: "The Guardian came out saying how wonderful it had done during the first couple of days, but we think it is down to just over 20% now."
Mullins predicted that sales of The Times in September would show an increase for the title.
"We certainly haven't been affected and our monthly sales will be up as much year on year as it has been for the past five months – the charge forward will be maintained," he said.
A Guardian spokesman would not comment on recent performance, but said expectations had been exceeded in the first week.
"Our objective was to put the paper into the hands of as many people as possible, and we have succeeded.
"We are confident that we can continue to maintain this momentum," she added.
Meanwhile, The Guardian is to embark on a massive sampling exercise for the new Berliner format.
The paper's marketing director Marc Sands said around 3.1 million homes and businesses around the country will be on the receiving end of a single-page door-drop in October.
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