Flights departing from Heathrow Terminal 3 were temporarily grounded due to a security alert.
A "suspicious item" was found by an X-ray scan of baggage at the west London airport on Friday morning, the Metropolitan Police said.
Tim Chambers, a passenger on a British Airways flight to Bilbao, Spain, told the Press Association his plane was held on the tarmac for two hours due to a suspected "security lapse", with each departing aircraft being searched by armed police.
Mr Chambers said the captain on his flight told passengers the delays were due to an incident involving a man said to have grabbed his bag and run through security after it had been pulled aside to be checked.
As a result, all flights from Terminal 3 were grounded, he said, citing the captain.
While it was initially feared the passenger had run back on to the airside area of security from arrivals, it was later confirmed this was not the case.
The disruption resulted in long queues at the airport, with some passengers complaining of a lack of information and others fearing they may miss their flights.
A Metropolitan Police spokesman said: "Police were called at approximately 8am to Heathrow Terminal 3 following a report of a suspicious item detected by an X-ray of luggage.
"Officers are working with the airport authorities and inquiries are continuing."
He added that no arrests had been made.
An airport spokeswoman said: "Departures were suspended for a short time from Terminal 3 due to a security issue.
"We are very sorry to any passengers whose journeys are affected. The safety and security of our passengers and colleagues is paramount."