The world's largest advertising agency has loaded pressure on Google by telling it to apologise to customers and advertisers who saw inappropriate content on YouTube, Sky News has learned.
WPP (Frankfurt: A1J2BZ - news) , through its media-buying company GroupM, has also written to clients explaining the potential risks to their brands, and offered them the choice of what they wanted to do next.
Google was summoned to appear in front of Cabinet Office ministers on Friday.
Sky News understands that the company apologised to senior civil servants representing the Government and pledged a review of their advertising systems.
Google was asked to return for another meeting next week to set out the action they plan to take.
GroupM buys advertising space for a range of international companies, on a variety of media. It is responsible for more than $100bn a year of spending, making it the biggest media buying agency in the world.
Sky News understand the company has recently offered "advisory notices" to clients, warning them of the dangers of certain "uncurated" platforms - such as Snapchat Lenses.
It has not, so far, advised companies to pull their advertising, but has told them they should reflect on their "attitude towards risk" and decide if they want to change their strategy.
GroupM expects responses from major companies in the next 48 hours and then will decide on what action to take.
It is the attitude it takes towards YouTube, which is owned by Google, that will make the greatest impact.
Rob Norman, GroupM's chief digital officer, told Sky News that Google's response "has so far not been sufficient" and he said it needed to make "a public announcement apologising to consumers for the presence of the content, and telling those consumer that they should never imply that an advertiser endorses that content.
"The company needs to publicly apologise to companies whose reputation has been compromised, and to take action," he said.
"Google is presently saying that it takes this issue seriously, but what we will want to see is action.
"If nothing happens then I think the attitude of the market will harden."
Yvette Cooper MP, chair of the Home Affairs Select Committee, has said that Google's failure to remove the hate videos by was "frankly astonishing"
On Tuesday, the Home Affairs Select Committee summoned bosses from Google, Facebook (NasdaqGS: FB - news) and Twitter (Frankfurt: A1W6XZ - news) , to question them about the action the web giants were taking to remove hate speech from their platforms.
At that appearance, the committee drew attention to videos by National Action, a proscribed organisation which the Government has said is linked with terrorism.
Nevertheless, videos from the group remain on YouTube. The committee said it had found one in which masked men shout "they fear us because they think we will gas them, and we will."
The committee also raised the issue of adverts automatically being put next to hate videos.
Reports on Thursday and Friday in the Times found that government and major brand advertising was still being hosted next to hate videos.
In Friday's letter, Ms Cooper also wrote: "Google is the second richest company on the planet. The lack of effort and social responsibility it is showing towards hate crime on YouTube is extremely troubling.
"It is inexplicable to us that Google can move very fast to remove material from YouTube when it is found to be copyrighted, but that the same prompt action is not taken when the material involves proscribed organisations and hateful and illegal content."
A Google spokesman said: "We have strict guidelines that define where Google ads should appear, and in the vast majority of cases, our policies work as intended, protecting users and advertisers from harmful or inappropriate content.
"We accept that we don't always get it right, and that sometimes, ads appear where they should not. We're committed to doing better, and will make changes to our policies and brand controls for advertisers."