。
Staying silent will be regarded as de facto consent.
Literal
Be-silent-is [when-と] de-facto [possessive-の] approval [quotative-と] is-regarded.
事実上 ('in actuality, de facto') is a fixed Sino-Japanese adverbial used in formal and legal contexts — 'effectively, in fact.' みなす ('to deem, to regard as') is a verb almost always paired with quotative と to mark what something is being considered as — 'is regarded as X.' The whole thing is in passive (みなされる) and polite-form, befitting an official warning.