Use ～に違いない to assert a conclusion you are confident of on the basis of evidence or reasoning — "it must be that…, no doubt…". It is stronger and more subjective than ～はずだ (which relies on shared expectation) and far stronger than ～だろう. In reports, email and negotiation it appears as the polite ～に違いありません; in speech and internal discussion the plain form is common, but note that pressing a client with it can sound presumptuous, so ～のではないでしょうか is often safer.