This is a new one on me — I seem to have just come across it when people are thinking of changing the rule, too. There’s no real consensus among authorities but many still recommend that, in formal writing at least, when “likely” is used as an adverb, it calls for a modifier, such as “very” or “quite.” “Probably,” on the other hand, can stand alone. However, the main reason may be to avoid arguments with sticklers.
American Heritage Dictionary of the English Language:
Used as an adverb likely is most commonly preceded by a modifier such as very or quite: He will quite likely require some help with his classes. But the unmodified use of likely is common enough in educated writing, and though it might be better avoided in highly formal style, it should not be regarded as incorrect: They’ll likely buy a new car this year.
Merrill Perlman has an interesting post on this issue in the Columbia Journalism Review. Here’s her conclusion:
“Garner’s Modern American Usage, which did not separately address the question until the third edition, in 2009, says that “it is common to use likely as an equivalent of probably and not to insist on the modifier.” At Stage 5 of the Language-Change Index, Garner’s says, it is “fully accepted,” “(not counting pseudo-snoot eccentrics).”
There are apparently lots of those still out there, though, because the objections surface regularly. This will “likely” sound like a broken record, but here’s the best advice on the use of “likely”: Careful writers “most likely” should use “probably” in its place if they think their readers will “likely” object to “likely” without a modifier.”
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