Britons have been warned to leave an extra four hours if travelling into the European Union (EU), with officials having no idea which European airports will fully implement new travel red tape.
Brussels and EU countries refused to tell the UK Government where the new "Entry/Exit System" (EES) will be enforced.
The unknown enrollments risk Britons facing huge queues at European airports, with travellers told to give themselves an extra four hours.
The EES requires Britons and other non-EU citizens to register at the EU border by scanning their passports and taking their fingerprints and photos at electronic booths.
Passengers may have to answer several questions, including their travel plans, where they are staying, travel insurance and whether they have enough money to cover the trip.
Refusing to answer any of the questions could result in being questioned by border guards.
Information about the project has been very limited, leaving lots of Britons in the dark.
Some countries, including Estonia, Luxembourg, and the Czech Republic, have publicly announced they will implement EES for all arrivals and departures from day one.
The Entry/Exit System machine has been rolled out today
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Spain will only test the EES with a single flight arriving at Madrid airport on day one of the system.
A full list of locations as to where EES will be rolled out has not been provided.
British passport-holders will need to register for the EES upon their first visit to a country where checks are operating.
It will be a requirement when entering EU countries within the Schengen "open borders" zone, as well as Iceland, Norway, Switzerland and Liechtenstein.
Those with dual nationality can only register for the system under one passport.
When leaving a European country, travellers only need to scan their passport and provide either a photograph or a fingerprint at the border.
A Home Office spokesman said: "Whilst the checks should only take one to two minutes for each person, they may lead to longer wait times at border control upon arrival in the Schengen area.
"At the juxtaposed ports, where registration will be completed in the UK prior to departure, there may be longer waits at busy times."
Britons have been told to leave themselves an extra four hours
| PAThe EES will be phased in gradually over six months, up until April 2026.
EES was due to be implemented in 2022, but has since been delayed five times before its roll out today.
Checks will also be carried out in the UK at ports which already have "juxtaposed" passport controls with Belgium, the Netherlands and France.
EES requirements at the Port of Dover and Eurotunnel will initially only apply to HGVs and coaches.