Heathrow's Terminal 4 has been evacuated after fire alarms went off in the second major alert at the airport within days.
Staff began evacuating passengers shortly after 1pm.
Sunita Jaswal had just boarded an Air Serbia flight to Belgrade with her partner when the alarm was raised.
She told The Independent: "We were just boarding when the alarms went off.
"Staff waited 10 minutes while the fire alarm went off because the doors were automatically locked and only British Airports Authority (BAA) could open them.
"We're on the runway now and I can see a member of staff being told off by a boss, who is saying 'you could have sent the passengers to where the fire was'.
"Staff directed us to one exit, then another. It was farcical. In light of recent security issues, this is beyond negligent.
"I doesn't seem like there was a fire, but if there had been the staff's lack of knowledge on what to do could have cost lives."
Confirming the evacuation, a spokesperson for Heathrow initially said the matter was being investigated "as a matter of urgency".
The spokesperson later told The Independent there was no fire.
Around 27,000 passengers a day travel through Terminal 4. Airlines include Air France, El Al of Israel, Gulf Air, KLM, Malaysia Airlines and Qatar Airways.
This is the second evacuation of a terminal at Heathrow in three days; on Tuesday, thousands of passengers were evacuated after a fire alarm was activated. Dozens of flights were delayed.
Passengers reported being stuck on the runway following the alert and others were stopped from boarding planes as the incident was investigated.
On Wednesday a suspicious object at the hold baggage screening area at Manchester airport triggered the evacuation of Terminal 3 at that airport, with many flights heavily delayed or cancelled.
