Aer Lingus is boosting capacity on its Belfast-Heathrow route after images emerged showing a packed aircraft.
Passenger Sean Mallon took the photos on the London-bound flight, which showed travellers sitting side-by-side.
Health Minister Robin Swann has said he was shocked at the image and called for Aer Lingus to review its processes.
The carrier said on Tuesday it was adding an extra flight on each day of operation to reduce passenger numbers on each aircraft.
Aer Lingus also advised passengers to give themselves extra time at the airport as the check-in process would take longer to allow for the increased number of bags being put in the aircraft's hold.
"The safety and security of Aer Lingus' customers and crew is our number-one priority, and these process changes are being implemented as a matter of urgency."
'Living in a time before Covid-19'
Speaking shortly before the Aer Lingus statement was issued, Mr Swann said: "I was shocked at the images of Aer Lingus.
"It is as if they were living in a time before Covid-19."
It was as if the airline was "completely oblivious to any of the regulations, social distancing, or guidance that has been enunciated and published by our executive here and the government in Westminster", said the health minister.
Speaking at the daily Stormont press briefing on Tuesday, Mr Swann called on the carrier to revise its approach.
Earlier, Belfast City Airport Chief Executive Brian Ambrose said there was no clear guidance on social distancing measures on an aircraft.
The coronavirus crisis has seen NI flights suspended, apart from two London services operating from Belfast City and City of Derry airports.
Speaking on the BBC NI's Nolan Show on Tuesday, Mr Ambrose said social distancing measures were in place in the airport, but the issue arose after "the first big influx of passengers" on a flight since the pandemic.
Mr Ambrose said it was not yet clear whether the issue was inadequate measures or if people were not adhering to the measures.
"For the month of April the average loads on our flights were about 20% to 30%, so it hadn't become an issue with large volumes of people on a single flight," he said.
"Then on Monday there were 154 passengers on a 174-seater aircraft," he added.
'You cannot square this circle'
He said he had immediately been made aware of a number of "pinch points" by his operations director.
"We are putting more staff on the ground and making sure there is more compliance and customers adhere to it," he said
Mr Ambrose said there was no clear guidance on social distancing measures on an aircraft.
"It is not clear to anyone within the industry how you can ever achieve social distancing in an aircraft," he said.
"The reason the government has not been definitive is you cannot square this circle.
"You can't keep people two metres apart in an aeroplane.
"You are in a closed environment where there is a mixing of air."
He said even keeping the middle seat empty did not meet "two-metre" social distancing rule, which he said may be futile on an aircraft in which air is mixed.
Mr Ambrose said Monday morning and Thursday evening flights out of Belfast City would now become busier with the reopening of the construction industry.
The International Air Transport Association (IATA), the body representing global airlines, said it was opposed to measures such as leaving the middle seat empty, saying this would not be financially viable for carriers.
"Nobody has demonstrated that having the middle seat empty reduces the chance of transmitting Covid-19 from one person to another," said IATA's medical advisor, David Powell.
However, it is in favour of passengers wearing facemasks.
Virologist at Queen's University, Conor Bamford, said that on a plane or in an airport "close connections with people would make it very straightforward for the virus to spread".
"The virus would be spread more from talking or breathing rather than recirculation of air in a plane," he told The Nolan Show.
"We might be lucky as there is not that much virus out there In the community but it is certainly not a risk you would want to be taking at this stage," he added.
In a statement on Monday, Aer Lingus 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."
North Antrim DUP MP Ian Paisley, who also flies to London for work in London, said it would be the "height of lunacy" to cut flights between Northern Ireland and Great Britain to help combat the spread of the virus.
The Health and Safety Executive (HSE) NI said the health and safety on board any flight does not fall within its remit.
"HSENI are looking at the other issues raised in relation to following the public health guidelines in the airport before boarding the flight," a spokesman told the BBC.