taru-adjective, adverb taking the 'to' particle
Bound form used as a taru-adjective or adverb with と; mainly appears in formal or literary contexts, often in the phrase 確たる証拠 (firm evidence).
See also: 確たる証拠
確たる証拠がない限り、彼を疑うべきではない。
As long as there is no firm evidence, we should not suspect him.
彼の主張は確とした根拠に基づいている。
His claim is based on definite grounds.