Prior to
From 'Vocabulary' part of The ABC Of Plain Words by Sir E Gowers (1951)

Do not use prior to as a preposition instead of before. Before is simpler, better known, and more natural, and therefore preferable. It is moreover at least questionable whether prior to has established itself as a preposition. By all means use the phrase a prior engagement, where prior is doing its proper job as an adjective. But do not say that you made an engagement prior to receiving the second invitation.

Mr. X has requested that you should submit to him, immediately prior to placing orders, lists of components . . .

Sir Adrian Boult is resting prior to the forthcoming tour of the B.B.C. Symphony Orchestra.

In sentences such as these prior to cannot have any advantage over the straightforward before.

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