In the United States a percentage (numerical - 50%) is always one word. I have read two crime novels by a British author who breaks the word up, "...there is 25 per cent chance that serial killers follow a ritulistic pattern."
I have checked two dictionaries on my iPad and both explicitly show per cent and percent as British and American spellings respectively. This answer from English Language StE claims:
Both spellings, percent and per cent, are acceptable, but consistency should be maintained. Percent is more common in US usage; per cent is more common in British usage.
If you read that page, you will find ngram data supporting that the English usage is increasingly affected by the American one. This explains Spagirl, who has never used the canonical form.
I'm British, and would certainly use the two word "per cent" rather than "percent," so I would hazard a guess that two words is the way all us Brits spell it.