"Cancelled" or "Canceled"?
The past tense of cancel is strictly cancelled in British English (BrE). In American English (AmE), however, it is spelled (BrE spelt!) canceled. Note that cancelled is also acceptable in American usage. There are many other verbs whose past tenses and present participles follow a similar pattern:
- worship: worshiped (AmE)/worshipped (BrE) • worshiping (AmE)/worshipping (BrE)
- travel: traveled (AmE)/travelled (BrE) • traveling (AmE)/travelling (BrE)
- label: labeled (AmE)/labelled (BrE) • labeling (AmE)/labelling (BrE)
- libel: libeled (AmE)/libelled (BrE) • libeling (AmE)/libelling (BrE)
- devil: deviled (AmE)/devilled (BrE) • deviling (AmE)/devilling (BrE)
There are some notable exceptions in which the last consonant is always doubled in the past tense and present participle. Examples:
- compel: compelled • compelling
- corral: corralled • corralling
- repel: repelled • repelling
- refer: referred • referring
- occur: occurred • occurring
- demur: demurred • demurring
- whip: whipped • whipping
- fit: fitted • fitting
And there also those words whose last consonant is never doubled when forming the past tense or present participle. Examples:
- differ: differed • differing
- succo[u]r: succo[u]red • succo[u]ring
- solicit: solicited • soliciting
- gallop: galloped • galloping
A special example is the verb program[me]:
- (AmE) program: programed/programmed • programing/programming
- (BrE) programme: programmed • programming
See the number of occurrences of cancelled and canceled from the American Corpus and British Corpus below. This supports what @JoseK wrote as comment to the question:
Corpus of Contemporary American English (COCA):
CANCELED 3746
CANCELLED 640
British National Corpus:
CANCELLED 1194
CANCELED 1