Google has been asked to explain itself after Government adverts were placed on "inappropriate and hate-filled sites".
The technology giant has been accused of "profiting from hatred" by MPs over its YouTube advertising system that saw taxpayer-funded adverts appear alongside extremist content on the video site.
The Home Affairs Select Committee has now written to Google asking for an explanation.
The Cabinet Office said it has placed a temporary restriction on its YouTube advertising "pending reassurances from Google that Government messages can be delivered in a safe and appropriate way", while The Guardian, Channel 4 and the BBC have also halted their advertising with the firm.
Google's advertising network uses an automatic filtering system which places adverts on websites and videos.
In a letter to Google's communications vice president Peter Barron, committee chairwoman Yvette Cooper said: "Government advertisements and major brands advertising is still being placed on inappropriate and hate-filled sites.
"As a result Google and these organisations are still profiting from hatred.
"In addition to explaining to the Government and to your advertisers how this has happened and what you are doing to prevent it ever happening again, please can you provide the Committee with a full explanation of this, including whether you will be refunding money to the Government and other advertisers."
The internet giant's UK managing director Ronan Harris had earlier admitted the company "can and must do more" to combat what it called "bad advertising".
"We've begun a thorough review of our ads policies and brand controls, and we will be making changes in the coming weeks to give brands more control over where their ads appear across YouTube and the Google Display Network.
"With millions of sites in our network and 400 hours of video uploaded to YouTube every minute, we recognise that we don't always get it right.
"In a very small percentage of cases, ads appear against content that violates our monetization policies," he said.
"We promptly remove the ads in those instances, but we know we can and must do more."
However, Ms Cooper said the company's "lack of effort and social responsibility" towards the content was "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," she said.
"The Committee expects to hear from you on how you are using some of YouTube's very significant revenue to put this problem right by devoting sufficient resources to ensure that vile and illegal material is removed proactively from your platforms, and that neither you nor those that create these videos profit from hatred."
Media advertising firm GroupM has suggested "a 100% foolproof system" to prevent further incidents may not be possible.
The firm, which is part of advertising giant WPP, also warned that Google and YouTube's tools to stop bad advert placement were "not infallible" in a letter to clients.
GroupM's chief digital officer Rob Norman said the technology giant should apologise over the incident.
"We believe Google owes two apologies, one to advertisers for compromising their brand reputations and the other to consumers for the presence of the content," he said.
"Google should also make clear that the flaw is in Google's controls, so no consumer may think that a brand, through its adjacency, is an endorser of the content."