Both of them are correct. It depends on whether we should use the definite article the before utmost or not.
If you don't use utmost, it should be of importance as importance is a mass (abstract) noun which doesn't require an article.
However, if you use utmost, you could use the the because utmost is in the superlative form meaning greatest or most extreme.
According to Oxford Online Dictionary, there is an example of "a matter of the utmost importance" and according to Merriam-Webster, there is "a matter of utmost concern".
They show both of them are correct.
As FumbleFingers has shown in the comment, the linked Ngram Viewer shows there is no big difference in their usage in American English. However, British English seems to favor of the utmost importance according to this Ngram Viewer.