EasyJet could face legal action after doodling adverts in chalk on Belfast pavements.
The budget airline has commissioned three chalk-drawn ads, in its trademark orange livery, on footpaths in the city centre.
But the firm didn't ask permission and is facing an inquiry by the Department for Regional Development, which is responsible for footpaths.
The department could bill EasyJet for the cleaning, force it to remove the ads, or take legal action.
An EasyJet spokeswoman has told the Irish News the pavement advertising is an attempt to be more intrusive.
The spokeswoman says the adverts were drawn using chalk and the company understood it did not need permission if the designs were temporary.
She added: "As far as we know this is the first time in Northern Ireland that something like this has been done and we will be doing it over the next couple of weeks in various places."
A spokesman for the Department of Regional Development says the matter is under investigation.
Belfast Deputy Lord Mayor Frank McCoubrey said: "We are trying to boost the image of Belfast and to bring in tourists, not to have large companies come along and do whatever they want to."