The Antarctic ozone hole would have been much bigger by now if ozone-depleting chemicals had not been banned in the 1980s, according to research.
The study found that the Antarctic ozone hole would have grown in size by 40 percent by 2013, and the ozone layer would be thinner over middle latitudes of the northern hemisphere.
A large hole would have opened up over the Arctic, big enough to affect northern Europe, the research said.
The Montreal Protocol, regarded as one of the most important global treaties in history, was signed in 1987 after the discovery of a hole in the ozone layer above the Antarctic, the part of the upper atmosphere where ozone is found in high concentrations.
Ozone absorbs ultraviolet radiation, preventing most of it from reaching the ground.
The UN agreement phased out ozone-depleting chemicals, including chlorofluorocarbons (CFCs) - once widely used in fridges and spray cans.
The new research, published in the journal Nature Communications simulated what the ozone hole would have been like today if nothing had been done.
The ozone loss would have led to increases in UV levels of about 10 percent in Australia, New Zealand and the UK, leading to more skin cancers, the study concludes.
Models project a gradual increase in stratospheric ozone with the Antarctic ozone hole expected to disappear by about 2050.
Arctic ozone hole avoided
Models showed that not only would the Antarctic ozone hole have been much larger, but that at certain times, a large hole would have opened up at the other end of the globe. This would have rivalled that of the Antarctic and would have affected northern Europe.
''We would be living in an era of having regular Arctic ozone holes", said lead researcher, Martyn Chipperfield of Leeds University.
He said there would have been ozone depletion over mid-latitudes where there are high populations, including parts of Europe.
Jonathan Shanklin of the British Antarctic Survey is one of three Cambridge scientists who discovered the ozone hole 30 years ago this month.
He said the Montreal Protocol was the UN's most successful treaty to date and observations from the British Antarctic base were showing signs of a ''recovery'' in ozone levels.
''The protocol provides a lesson for the future and we must hope that the coming climate change talks show the same foresight and result in a treaty that will benefit the whole planet,'' said Dr Shanklin.
Since the Montreal Protocol came into force, levels of chlorine and bromine containing ozone depleting chemicals have peaked and then declined.