The Danish business, which offers outsourced services like cleaning and catering, is co-owned by pan-European private equity firm EQT and Goldman Sachs.
An ISS spokesman said: “An IPO remains an option for the company, and exploratory work is being undertaken. However, no final decision has been made.”
ISS has appointed UBS and Goldman Sachs to explore the options, but a float in Copenhagen is still more likely than London.
The company was close to being snapped up two years ago by UK contracting giant G4S for £5.2bn before the deal fell through.
Any float is likely to value the company in the same region.
In August the company appointed former ITV boss Sir Charles Allen as chairman, sparking speculation a float was imminent.