Google boss Larry Page is beginning to sweat.

The search giant, its massive global ad machine under attack, apologized on Monday for allowing clients’ marketing efforts to appear on inappropriate YouTube videos.

“This is a good opportunity for me to say sorry, this should not have happened, and we need to do better,” Matt Brittin, Google’s head of business and operations in Europe, the Middle East and Africa, said at an advertising conference in the UK.

Google’s ad sales, expected to grow 16 percent this year, to $28.6 billion, have come under pressure in recent days after it was learned that ads for a number of large brands appeared on videos by controversial Muslim preacher Wagdy Ghoneim, white nationalist David Duke and terrorist support groups.

Several large advertisers said YouTube parent Google had better police the matter or they will pull their advertising dollars.

WPP Group executive vice president of brand safety John Montgomery told The Post that major advertisers are on “high alert” after learning of the brand safety issue.

One ad holding company, Havas, yanked all its clients from YouTube UK as a result.

“We want them to do more, to use their considerable technical and financial expertise, to fix the problem … clients have a limited tolerance for risk,” Montgomery told The Post.

Google’s robust digital ad revenue growth has it gobbling up 78 percent of total US search ad revenue, according to eMarketer. Display ads are another $5 billion business for Google.

On Monday, Wall Street analyst Brian Wieser at Pivotal Research downgraded Google parent Alphabet to hold on fears it might face some pressure on ad revenue.

Wieser wrote that Google’s comment Friday that it couldn’t guarantee that any ad is classified quickly enough or with the correct filters “is a big deal.”

“Google will probably need to articulate goals that sound more like a zero tolerance policy, to alleviate concerns, before it can fully recover,” Wieser said.

Alphabet shares slipped $3.72 on Monday, to close at $848.40.

Some advertisers create white lists to protect where their ads appear online, one digital ad executive told The Post.

“The Google exchanges let just about anyone on,” said a person with knowledge of the automated ad business.

Google said it has been reviewing its ad placement procedures but will now accelerate that process.

Some argue that Google doesn’t want to wade into free-speech issues that put its profits at risk.

But that did not sit well with one advertising insider.

“Barnes & Noble isn’t going to put hard-core porn next to Proust. Free speech is a convenient cover. There’s a profit motive, it’s very expensive and time-consuming.”

Some Google rivals note that the firm could hire more data scientists to police their content instead of serving up ads attached to, for instance, jihadists or neo-Nazis running websites.