BBC

After 9/11, US authorities demanded that airlines should provide personal passenger data for all inbound flights.

But the subsequent US-EU agreement was ruled illegal by the highest European court in May of this year. Saturday was the deadline for a new deal.

A European Commission spokesman said that a legal black hole could be created by the lack of agreement.

"There is no agreement. There is a legal vacuum as of midnight tonight," EU Transport Commission spokesman Jonathan Todd said on Saturday.

As a result, airlines refusing to provide passenger lists to the US may lose landing rights in the country, but those that do risk facing legal action under EU member states' data protection legislation. Read on...

BBC provided a little more specific reporting of the information the US demands:

Information about the booking of the ticket, about the passenger, and some things about his or her travelling companions and travelling history.

Frequent flyer miles, credit card number, e-mail address and telephone number, could all be present, depending on how the ticket was booked and paid for.

Requests for a special meal will show up too, unless they indicate the passenger's race or religion. In this case, the CBP computer is meant to filter out the data.

There are also "open text" fields where the travel agent may enter general information about a passenger's tastes and preferences, but the CBP computer is meant to scan these only for data relevant for security purposes. Manual searches are only allowed if the passenger has been identified as "high-risk".

More detail here