By Daniella Relph BBC News, Washington DC

Modern EU-issue passports carry machine-readable data

The United Kingdom, Australia and Japan are among those countries affected by the heightening of security measures at US airports.

Machine-readable passports are now standard issue in most countries.

They contain lines of coded data usually beneath the passport photo that can then be swiped at airport check-in and at US immigration.

The data allows the information on the passport to be immediately cross-checked against databases run by the US security services.

Warning letter

Those visitors who do not have the correct type of documentation will be issued with a one-time exemption.

They will be given a letter explaining their documents need to be changed and their passport will be stamped to show they have been warned.

Most countries in the European Union are affected by the new rules as well as Australia, New Zealand, Japan and Singapore.

The changes are being driven by the US Department of Homeland Security and the Department of State.

They say they need as much information as possible on every visitor to the US to help ensure the safety of American citizens.