Aer Lingus has said it is reviewing procedures on its Belfast to London flights following a claim it was not observing social-distancing measures.
Passenger Sean Mallon took photos on a flight to Heathrow on Monday, showing most passengers sitting close together.
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.
'95% of seats taken'
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's Evening Extra programme.
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."
Cabin social distancing 'impossible'
The only other route operating from Northern Ireland is Logan Air's service between City of Derry Airport and London Stansted.
In other developments on Monday:
- The leaders of Northern Ireland's four main Christian Churches say now is not the right time for a return to collective worship
- Economy Minister Diane Dodds calls for more "national support" for the higher education sector
- One of Northern Ireland's largest house builders, Hagan Homes, resumes work on eight of its sites
- Reduced hot-desking and alternatives to social distancing where it is not possible, are among measures being considered to let workplaces reopen.
Both benefited from a government package worth £5.7m to support remaining NI flights during the pandemic.
Loganair's chief executive, Jonathan Hinkles, said conversations were continuing about how to move on after lockdown, but that social distancing on board was impossible, due to:
- The economic cost
- The air conditioning systems on aircraft that recycle air in the cabin, which Mr Hinkles said defeated the purpose of social distancing
Instead, Loganair has said it would provide passengers with masks - something other airlines will have to think about.
The European Commission is preparing to unveil a set of rules for the safe reopening of air travel when the lockdown ends.
Measures being considered include compulsory face masks, disinfection of planes and social distancing requirements in planes and airports.