- doubtless, no doubt, undoubtedly, doubtlessly1. Fowler (1926) rightly noted that doubtless and no doubt, used adverbially, convey probability rather than certainty about what follows, so that They are doubtless [or no doubt] guilty and No doubt he [or He doubtless] meant well connote no more than strong belief and reassurance respectively. If real conviction is intended, it is important to use undoubtedly or without (a or any) doubt, as is shown by substituting them in the example already given: They are undoubtedly [or without doubt] guilty.2. Doubtlessly, for long made unnecessary by the adverbial role of doubtless, is making a strong comeback, first in the US and then in Britain:
• The current argument…doubtlessly offers a cogent and easily understood explanation for the current deadlock in East–West relations —Washington Post, 1984
• The overall effect is of a sobriety that can…make even Britten's doubtlessly ‘greater’ War Requiem sound a mite theatrical —Independent, 1994.
Modern English usage. 2014.