What is the test for establishing substantial prejudice in a claim for wrongful dismissal?

Finally, plaintiff has not established substantial prejudice. Plaintiff argues that substantial prejudice exists because the driver's status as an employee was a complete defense to coverage, but this argument ignores the arbitrators' ruling that plaintiff's breach of the duty to defend barred it from attempting to prove that defendant's liability was not covered. To find substantial prejudice, we would have to disregard the arbitrators' interpretation and application of Gray v. Zurich, supra, 65 Cal.2d 263, and,

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