When its a private one. There are no private monopolies,the only monopolies are government State enforced protected ones. Even in a mixed economy with relative freedoms and lots of regulations and restrictions on business and trade monopolies cannot exist as witnessed by the fact that companies that are riding high one decade can suffer irreparable damage the next,whether by error, lack of innovation, complacency, taking its eye off the ball, to say nothing of the disruptive technological changes that have revolutionized the way consumers purchase products - see the radical churn in the high streets in recent years with large household names going under or being overtaken by upstart new kids on the block - and online.
State monoplies on the other hand are the real deal and like bureaucracies they endure eg the NHS, the BBC, State education system. There is no way anybody can compete with them as they operate outside the market, they are unchalengeable, untouchable; they stand there generation after generation, untouched by the winds of change, solid, stolid, adamantine, monolithic, Stalinist monuments to sloth, sclerosis, complacency, State patronage and corruption.
If the dictatorial illiberal stassi-like CMA were to apply the exact self- same rigorous standards and criteria as to what constitutes a monopoly to the Public sector as it does towards privately owned companies (which it has no business meddling with and interfering in whatsoever) then the NHS, the BBC, and the State education system would all have been broken up into separate parts a long long time ago.
But of course no such thing will happen because the State operates under a grossly hypocritical double standard when it comes to private enterprise and 'monopolies', a sort of political schizophrenia overtakes it, a double book keeping which reduced to Orwellian terms would be: Public Monopoly Good, Private Monopoly Evil.