All incoming flights were suspended at the Luton Airport in London over an alert sparked by the checked-in suitcase which was picked out by a security scanner.
Police said the terminal had been evacuated while the package was being examined.
"London Luton Airport has evacuated the terminal building after a suspicious item was found in the security search area. The decision has been taken with the police to ensure the safety and security of passengers and staff," an airport spokesperson said.
Britain had recently raised its terrorism alert to its second-highest level of "severe", implying an attack was imminent.
The UK government has said Islamic State (IS) in Syria and Iraq posed the UK's greatest-ever security threat.