Rip-off websites which charge people for free Covid-19 passenger locator forms feature prominently in Google search results, the BBC has found.
Airline Ryanair said passengers should be "extra vigilant" and only download forms from the official website.
The passenger locator form is designed to help airlines contact people in the event of a coronavirus outbreak.
Google said it had removed several false ads but the BBC found some still topped its search results on Wednesday.
"It's no surprise that these copycat firms have found yet another route to try and make money out of people for something that is free," said Guy Anker, deputy editor of consumer website Money Saving Expert.
While selling passenger locator forms is not illegal, Mr Anker said the rip-off websites were "devious".
Google's policy is to ban ads for unofficial websites selling government documents. But in May, the BBC reported that rip-off ads for travel visas, driving licences and other documents topped Google search results every time it checked during a 12-month investigation.
"If Google is being user-first, it would be in its users' best interests not to feature these ads and to make sure that the official site for passenger locator forms and other documents come near the top [of any search]," said Mr Anker.
On Sunday, one passenger said Ryanair staff had been warning customers at Hamburg airport after some had paid for the free forms.
A spokeswoman told the BBC: "Passenger locator forms should only be downloaded from an official government source or website and are always free of charge."
She said passengers should "avoid all non-government websites for such forms".
The UK Home Office said the form was part of its "enhanced borders regime" to reduce the risk of new coronavirus variants being transmitted.
"The form is free, easy to complete and only available via the gov.uk website," a spokeswoman said.
"The government does not endorse or recommend any third parties who provide passenger locator form completion services," she added.
In a statement, Google said: "We only allow governments or their delegated providers to advertise for official documents or services.
"When ads breach our policies we take action to remove them. Last year, our team blocked or removed over 3.1 billion ads for violating our policies."
However, on Wednesday, searches for "passenger locator form" on Google were still returning the rip-off ads.