Latin- Labor, laboris.
OF writing, labor, labour. The latter seems to have been most commonly used; in English writing of the 1300s labour was used.

In the U.S. Noah Webster, 1798, says "common sense and convenience," would lead to adoption of public for publick, favor for favour, etc. In 1806, he swept the 'ou', a 'silent' letter, out of his dictionary. Also redundant consonants; traveler for traveller, wagon for waggon, etc.
He also transposed the 'er' in calibre, to caliber, etc.

Shakespeare used honor and honour indescriminately

Johnson in England was firmly for retaining the 'u' ih honor, labor, etc.

Webster dominated the field in America, hence 'labor' in the U.S.

The above from H. L. Mencken, "The American Language."