The US government is taking aim at the engine of Google’s immense wealth – its extremely lucrative ad tech business.

A trial beginning on Monday will hear the Department of Justice’s case that the search engine’s parent company Alphabet illegally operates a monopoly in the market.

The company earned more than $200 billion (£152bn) last year through the placing and selling of ads seen by internet users.

Alphabet has argued its success is due to the “effectiveness” of its services – but prosecutors say it has used its market dominance to stifle rivals.

“It is a really important industry that grabs billions of consumer dollars every year,” said Laura Phillips-Sawyer, a professor at the University of Georgia School of Law.

“I think all consumers have an interest in this litigation.”

It is the second major antitrust case the tech giant has faced in the US.

In August a judge ruled its dominance of search was illegal, with the penalties Google and Alphabet will face as a result of that decision so far unclear.

According to the lawsuit filed by the Department of Justice (DoJ) and a coalition of states in 2023, Google dominates the digital ad marketplace and has leveraged its market power to stifle innovation and competition.

Google meanwhile contends it is just one of several hundred companies that facilitate the placement of digital ads in front of consumers.

It argues that competition in the digital ad space is growing, not contracting – citing increased ad growth and revenues for companies such as Apple, Amazon and TikTok as proof in a blog post responding to the DoJ’s lawsuit in 2023.

Both sides will present their cases to US District Judge Leonie Brinkema, who is expected to deliver a verdict.

The bench trial comes on the heels of a landmark decision last month in a different monopoly case brought by the Justice Department against Google.

Judge Amit Mehta ruled that Google acted illegally to squelch competition in its online search business.

“Google is a monopolist, and it has acted as one to maintain its monopoly,” he wrote.

During last year’s trial, Google said it dominated online search because it had a better product.

And the company is seemingly deploying a similar defence in the ad tech case.

When asked for a statement, it referred the BBC to its 2023 blog post, in which it states that “no-one is forced to use our advertising technologies – they choose to use them because they’re effective.”

Judge Mehta held a status conference on Friday as he begins the process of deciding on remedies for Google’s conduct.

“The DoJ clearly had a big win, and they’re going to ride that momentum,” Dan Ives, managing director at Wedbush Securities, told the BBC.

He said he expects those remedies to involve “business model tweaks, not a breakup” of the company.

Meanwhile, in Justice Brinkema’s courtroom, the arcane process that governs advertising technology could make the DoJ’s attempts to prove its case an uphill climb.

“We all use search. We all intuitively understand that product,” said Rebecca Haw Allensworth, an antitrust professor at Vanderbilt University Law School.

By comparison, advertising technology is “so complex that I think that’s going to be a real challenge for the government to make a clear, simple monopolisation argument here.”

The US is not the only country where regulators are unhappy with Google’s ad tech business.

On Friday, the UK Competition and Markets Authority said it believed Google was abusing its dominance in the ad tech industry, according to the findings of its initial investigation.

It said it found that Google used anti-competitive practices to dominate the market for online advertising technology – and the potentially unlawful behaviour could be harming thousands of UK publishers and advertisers.

A Google representative said the decision was based on a “flawed” understanding of the ad tech sector.

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Google uses anti-competitive practices to dominate the market for online advertising technology, a UK watchdog has provisionally found.

The potentially unlawful behaviour could be harming thousands of UK publishers and advertisers, an investigation by the Competition and Markets Authority (CMA) has warned.

It accuses Google of preventing rivals from “competing on a level playing field” with its own tech for the billions of pounds spent by UK businesses on online advertising.

Google said the watchdog’s findings were “flawed” and said it would respond.

According to the CMA, the vast majority of businesses use Google’s services when placing digital ads on websites.

Google maintains it has a strong business incentive to help UK firms thrive, and argues that advertisers choose to use Google because its products work well and help their businesses grow.

The watchdog will now consider representations from Google before deciding what action to take.

If Google is found to have broken competition law, the watchdog could impose a financial penalty of up to 10% of annual worldwide group turnover and issue legally binding directions to the firm.

“We’ve provisionally found that Google is using its market power to hinder competition when it comes to the ads people see on websites,” Juliette Enser, the CMA’s interim executive director of enforcement, said in a statement.

She pointed out that many businesses were able to keep their digital content free by using revenue from digital adverts, which reach millions of people across the UK.

“That’s why it’s so important that publishers and advertisers – who enable this free content – can benefit from effective competition and get a fair deal when buying or selling digital advertising space,” she wrote.

But Google’s vice president of global ads, Dan Taylor argued the search giant’s advertising technology helped websites and apps fund their content, and effectively reach new customers.

“The core of this case rests on flawed interpretations of the ad tech sector. We disagree with the CMA’s view and we will respond accordingly,” he wrote.

Google’s activities in ad tech are also subject to continuing probes by the US Department of Justice and the European Commission.

Competition economist Dr Cristina Caffarra, told the BBC that while the CMA’s statement of objections certainly presented “another headache” for Google, the regulator was merely “joining the club” of those who have already taken action.

“The UK is by no means some sort of pathfinder here,” she said.

The Department of Justice, state of Texas – which along with nine other states sued Google over alleged abuse of its ad tech dominance in 2020 – and the EU are all far ahead, Dr Caffarra added.

In 2023, EU competition regulators told Google it might need to sell part of its ad-tech business to address their concerns.

But the tech-giant has argued this would be a “disproportionate” step.

Separately, Google is seeking to appeal a UK court decision in June to allow a £13.6bn collective-action lawsuit against it to proceed.

The case alleges the search giant behaved in an anti-competitive way which caused online publishers in the UK to lose money.

Google has vowed to oppose the claim “vigorously and on the facts”.

Additional reporting by Liv McMahon