The UK government pulled its advertising on Friday after The Times of London revealed the UK government and many major brands had their ads appear with extremist content, hate preachers and anti-semitic material.
The apology from Matt Brittin, president of Google's Europe, Middle East and Africa division, came after United Kingdom banks HSBC and RBS, along with major retailer Marks and Spencer (M&S), chose to suspend their adverts from appearing on platforms such as YouTube.
This weekend, WPP, the world's largest advertising company, increased the pressure on Google.
"Making the problem potentially worse if it is not fully solved", Mr. Wieser wrote, "Google faces a hostile industry of media owners in Europe (many of them owners of print properties which have been negatively impacted by Google's successes in recent years) and we expect they will be all too happy to highlight future brand safety failings, negatively impacting brands".
"Whatever Google's editorial policy, advertising should only be sold against content that is safe for brands".
Marks & Spencer acted after the United Kingdom government, the Guardian newspaper, grocer J Sainsbury Plc and other marketers pulled advertising from YouTube.
"Google assured us they would fix this very quickly and so we've chosen to hold them accountable rather than pull our allocations", he said.
However, Brittin said Google was looking at better defining hate speech and inflammatory content, simplifying controls available to advertisers.
"We're not confident that this approach will be sufficient to remedy advertiser concerns", a statement from the group said. French advertising company Havas SA said it was removing certain clients' spots from the site after it failed to get assurances that they wouldn't appear alongside offensive videos. Changes to the company's advertising policies were announced after they appeared alongside offensive content, such as videos promoting terrorism or anti-Semitism.
British supermarket chain J Sainsbury Plc., whose ads appeared on videos posted by the white nationalist Polish Defence League, said it and its sister brand Argos would immediately suspend all Google advertising. "We need to stand back and not roll from one issue to another", he added.
Internet giants including Facebook and Google have historically absolved themselves of responsibility for some of the content hosted on their platforms by insisting they are "technology companies" rather than media outlets, meaning they can not be regulated and fined in the way that newspapers, books or magazines would be if they published the views of extremists.
The digital behemoth has promised to alter its technology and policies, offering more control to advertisers. But he declined to say whether Google would actively look for extremist content, instead of waiting for users to flag it, according to The Guardian.
"We've heard from our advertisers and agencies loud and clear that we can provide simpler, more robust ways to stop their ads from showing against controversial content", Google's UK Managing Director, Ronan Harris, said.