"utmost importance" is a correct and usable phrase in written English. It can be used to emphasize the extreme importance of something. For example, "The safety of our customers is of utmost importance to us.".
Of utmost importance was fire.
Family is of utmost importance.
"Vision is of the utmost importance.
That is of utmost importance.
"Obviously, privacy is of the utmost importance.
They remain of the utmost importance".
For Wales, our relationship with England is of utmost importance.
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Ha Thuy Vy
MA of Applied Linguistic, Maquarie University, Australia