Passenger Sean Mallon took photos on a flight to Heathrow on Monday, showing most passengers sitting nest to each other.
Almost every seat was occupied and there was “no social distancing whatsoever,” said Mr Mallon.
Aer Lingus said safety was its “top priority” and any necessary changes would be implemented urgently.
The Coronavirus crisis has seen all flights suspended, apart from two London services operating from Belfast City and City of Derry airports.
Mr Mallon, who was travelling to England for work, said airline staff did not offer any guidance on social distancing before or during Monday morning’s flight, and passengers were allowed to take their seats as normal.
“I would say there was about 95% of the seats taken on the flight,” he told the BBC.
Passengers had queued up and boarded the plane in the same way as they would have done before the pandemic, he said.
“The queues were down the steps and out on to the tarmac as they were before all this happened,” he added.
Passengers were not offered hand sanitiser or told of any extra safety measures as they boarded the plane, said Mr Mallon.
The only advice they were given was a reminder to wash their hands after landing.
In a statement, the airline said: “In light of the unexpectedly high loads on the Belfast – London Heathrow service this morning and the level of demand for the route, Aer Lingus is reviewing its processes and procedures applicable to the operation of this service.
“The safety and security of Aer Lingus’ customers and crew is our top priority and any process changes that are identified as being required will be implemented as a matter of urgency.”
The only other route operating from the North of Ireland is Loganair’s service between City of Derry Airport and London Southend.
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