In 2010, the US started charging UK travellers to use the Electronic System for Travel Authorisation (Esta).

Unofficial sites charging five times as much as the US government soon flooded the top of Google's search results, despite breaking Google's ad rules.

Now, after a BBC News investigation, Google says it is tackling the issue.

While unofficial Esta sites will still appear in the search results, they should no longer appear above the official website as advertisements when using the most common search terms.

Since it has used machine learning to address the issue, the ads do still show up for some search terms. Google says this will improve in time.

Why has this taken so long?

The official Esta website is run by the US Department for Homeland Security. It charges $14 (£10.70) for each Esta application.

But countless unofficial sites appeared at the top of Google search results by buying advertisements.

These unofficial sites charged more than $80 for an Esta application.

Google's advertising policies explicitly forbid "charging for products or services where the primary offering is available from a government or public source for free or at a lower price".

The company did take down ads that were manually reported by its users, but the same websites would soon reappear with a new web address.

It was a fruitless game of "whack a mole".

What has changed?

The BBC sent several unofficial Esta ads to Google and asked why they had been allowed to remain on the platform. One of the websites advertised on Google was charging $99 (£76) per Esta.

Google took the ads down, but others immediately filled the space.

An ad for this Esta site was also taken down

After the BBC supplied more of the unofficial Esta ads, Google said it would look into the problem. It later said it had been able to develop its machine learning process to wipe out the unofficial Esta ads.

Following the change, commonly used Esta search terms no longer carry ads for the unofficial services.

Some less common searches may still return ads while the algorithm continues to learn, but the most obvious ones such as "esta" should no longer show ads, it said.

Beyond Estas

Other countries including Australia and Canada also have travel permits similar to the Esta.

Just like the Esta, there are countless unofficial websites offering Australia ETA and Canadian ETA permits at inflated prices.

Ads for these services have also been prominent on Google. But the search giant said it would use the same machine learning systems to eliminate these too.

In a statement, Google said: "We know that people look to Google ads for information about where to get goods and services, so we are committed to ensuring that the ads they see are useful and relevant.

"We use a combination of algorithmic and human review to catch and remove bad ads; and we continue to update our policies and methods of enforcement."

It said it encouraged people to report ads that slipped through the system, so they could be manually reviewed.