The amount is the regulator's largest penalty to date against a company accused of distorting the market.

The ruling also orders Google to end its anti-competitive practices within 90 days or face a further penalty.

The US company may decide to appeal.

If it fails to change the way it operates the service within the three-month deadline, it could be forced to pay 5% of its parent company Alphabet's average daily worldwide earnings.

Google had previously suggested that Amazon and eBay had more influence over the public's spending habits and that the commission's views "failed to fit the reality of how most people shop online".

However, the decision could set a precedent that determines how the EU's civil service handles related complaints about the prominence Google gives to its own maps, flight price results and local business listings within its search tools.

Fast growth

Google Shopping displays relevant products' images and prices alongside the names of shops they are available from and review scores, if available.

Image caption Shopping results often push traditional links off screen when viewed on smartphones
The details are labelled as being "sponsored", reflecting the fact that, unlike normal search results, they only include items that sellers have paid to appear.

On smartphones, the facility typically dominates "above-the-fold" content, meaning users might not see any traditional links unless they scroll down.

Google also benefits from the fact the Shopping service adverts are more visual than its text-based ads.

One recent study suggested Shopping accounts for 74% of all retail-related ads clicked on within Google Search results. However, the BBC understands Google's own data indicates the true figure is smaller.

Seven-year probe

The European Commission has been investigating Google Shopping since late 2010.

The probe was spurred on by complaints from Microsoft, among others.

The rival tech giant has opted not to comment on the ruling, after the two struck a deal last year to try to avoid such legal battles in the future.

But one price comparison service has welcomed the fine.

"An entire industry has suffered because of Google's unlawful, anticompetitive behaviour, and it has become a genuine struggle for survival for the likes of [us]," the chief executive of Kelkoo Richard Stables told the BBC.

"At the same time, Google's abuse has raised costs for merchants, and it has meant higher prices for consumers and much more limited choice."

Although the penalty is record-sized, it could have been bigger.

The commission has the power to fine Alphabet up to 10% of its annual revenue, which was more than $90bn (£70.8bn) in its last financial year.

Alphabet can afford the fine - it currently has more than $172bn of assets.

Image caption The Shopping service has helped Google make ads appear more attractive
But one expert said the company would be more concerned about the impact on its future operations.

"If it has to change the appearance of it results and rankings, that's going to have an impact on how it can monetise search," said Chris Green, from the tech consultancy Lewis.

"Right now, the way that Google prioritises some of its retail and commercial services generates quite a lot of ad income.

"When you consider the sheer number of search queries that Google handles on a daily basis, that's a lot of ad inventory going in front of a lot of eyeballs.

"Dent that by even a few percentage points, and there's quite a big financial drop."

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