By Dominic Hughes BBC Sydney correspondent

Security has been stepped up at airports throughout the world

Three elderly passengers sparked a security scare at Australia's busiest airport in Sydney after wandering through a wrong door.

Embarrassed airport officials explained how the three travellers walked past a guard and into a secure area, causing the cancellation of flights and delays for hundreds of passengers.

The security scare closed one of the domestic terminals at Sydney's airport for two hours.

Airport officials say the guard on duty was distracted while he helped some other passengers.

The incident only came to light after one of the three passengers mentioned to cabin crew that he did not appear to have been through the usual security checks.

Once they heard what had happened, the alarm was raised and a security lockdown of the terminal was put into place.

Some passengers had to leave their planes and go through security checks again.

Four flights were cancelled and hundreds of travellers were delayed.

The terminal is run by the Australian airline Qantas.