In almost all cases, yes. You need to get the facts straight before you take any formal action or hold a hearing. The only real exception is when the facts genuinely aren't in dispute.
Skipping the investigation, or rushing it because the conduct looks obvious, is one of the most common reasons employers lose tribunal cases they should have won.
Every situation has its own details, and those details change the right answer. For a read on yours, take the free Employee Situation Check or book a call.
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How to handle this, in a couple of minutes
Samantha talks through the practical steps.
General guidance to help you decide your next step, not legal advice. Employment law changes, so always check the current position for your own situation.