Eye-scanning technology is being used at Heathrow Airport's busiest terminal to speed up immigration checks.
Passengers have their eyes scanned at an automated booth
UK citizens and some foreign travellers can now use the system to skip passport checks at terminal one.
Passengers who volunteer can pre-register a photograph of their eye then have their eye scanned on arrival at an automated booth.
Project Iris has been operating at Heathrow terminals two and four since last June.
Immigration minister Tony McNulty said: "It is important that the UK remains at the forefront of the latest technology in immigration controls.
"Secure and effective border controls are vital to safeguard our citizens against terrorism, serious and organised crime and illegal immigration, while at the same time facilitating entry for legitimate travellers."
Apart from British citizens, Project Iris is open to frequent visitors to the UK and foreigners with leave to remain in Britain.
The £2.86m technology works by photographing a passenger's iris patterns and storing the data in a database together with their passport details.
These registered travellers can then proceed through automatic barriers in the immigration arrivals hall.
Project Iris is due to be installed at Gatwick North and South, Heathrow terminal three, Stansted, Birmingham and terminals one and two at Manchester by the end of this year.
The project was temporarily shut-down after the London bomb attacks in July, then re-opened on 3 January.