The chief executive of the parent company of the Guardian has said the newspaper would consider preventing readers from accessing its content if take-up of ad-blocking software becomes widespread.

David Pemsel, the chief executive of Guardian Media Group, would not be drawn on the rate of ad-blocking among Guardian readers, but acknowledged that there were some “big scary numbers” in some markets, with reports of as high as 25% of readers.

“What is important is that in the end we need to make sure the economics stack up,” he said. “If more and more of those ad-blockers are introduced, we will be far more aggressive by saying ‘you are consuming our content for free, you can’t’, and turn it off.”

Pemsel said the publisher is already undertaking limited testing of a pop-up message asking readers who are identified as using ad-blockers to switch them off.

“I think we were slow to this because we were so conscious about how to talk to our readers,” he said, speaking at the FT Digital Media conference in London. “We are now testing in certain environments very extreme graphics [with a message asking readers not to use an ad-blocker]. You can whitelist, pick certain brands, content distributors and say ‘I don’t want the ad-blocker to be served there’.”

As part of a trial, a small number of users who are accessing stories and blocking ads are being asked to whitelist the Guardian or become a member.

In October, the London daily financial freesheet City Am was the first UK newspaper to test banning readers, following Germany’s Axel Springer, the owner of Bild, Europe’s top-selling tabloid, which asked readers to switch off ad blockers or pay a €2.99 (£2.23) monthly fee to browse the website.

Pemsel also added that the Guardian’s coverage of the Panama Papers has helped to boost sales by 10,000-15,000.