A photo has emerged of DUP MP Sammy Wilson not wearing a face mask on public transport in London.
It appeared on social media on Thursday morning, showing Mr Wilson reading a magazine, as he headed to the airport.
A witness claimed he did not wear a mask for the duration of his tube journey from Westminster to Heathrow.
Mr Wilson said he accepted he "should have been" wearing a face covering and he would "accept whatever consequences there are".
"I have accepted that I wasn't wearing a mask, I should have been," he said.
"I'm making no excuses and will accept whatever consequences there are."
'Listen to the laws'
DUP leader Arlene Foster said "everyone is subject to the law and everybody is equal under the law".
"Therefore it is very important that we listen to the laws and abide by them," she said.
Deputy First Minister Michelle O'Neill said she had heard about the incident and the executive was "strongly encouraging" people to wear masks.
The man who took the photo - who asked not to be named - said Mr Wilson got on the tube at Westminster at about 08:30 BST and sat opposite him.
He claimed Mr Wilson did not wear a mask during the journey.
The DUP MP told the Belfast Telegraph newspaper: "I wore a mask before getting on the train, while on the train, in the airport and on the plane.
"I took a call and you can't talk through a mask as you are muffled, so I took it off but put it back on again soon after."
He said there were others in the carriage not wearing a mask.
Alliance deputy leader and North Down MP Stephen Farry later tweeted a photo of himself wearing a mask on the underground and said "most NI MPs do acknowledge, understand and respect rules" around face masks on public transport.
SDLP South Belfast MP Claire Hanna said Mr Wilson needed to "wise up".
Transport for London (TfL) says those caught not wearing a mask could face a minimum fine of £200, which doubles each time a person is caught, up to £6,400.
Exemptions apply for age, health and disability reasons and TfL provides an "exemption card" for those unable to wear one.
The witness told BBC News NI he had been contacted by the Met Police who said they had passed on the case to the British Transport Police.
A British Transport Police spokesperson said: "Face coverings are a mandatory requirement on public transport.
"If officers are on patrol and see someone not wearing a face covering on a train or at a station, they will begin by engaging with them and reminding them of the reasons why the protections are a necessary and lawful requirement.
"In these cases, the overwhelming majority of people will listen and comply without the need for enforcement. In other cases, passengers may have a reasonable excuse or a valid exemption from wearing a face covering."
Support local business. You can’t eat🍦when you’re muzzled! pic.twitter.com/sc4GHzs8ix— Sammy Wilson MP (@eastantrimmp) July 31, 2020
Mr Wilson has previously criticised the rules around wearing face masks.
In July, before face coverings in shops became mandatory, he posed on Twitter at an ice cream parlour in Islandmagee, County Antrim.
His message was "Support Local Business" and he went on to point out that "you can't eat ice cream when you're muzzled!"
He has also been critical of the government's handling of the pandemic and on Wednesday, he criticised what he called the "scare tactics" being used by Downing Street in relation to Covid-19 statistics.