Please accept my unreserved apologies.

Meaning

A formal and complete apology, leaving no room for doubt.

Examples

• John: 'This is completely unacceptable; I expect a full explanation.'<br>
• Alice: '<b>Please accept my unreserved apologies</b>, John. A full investigation is underway.'
• John: '<b>Please accept my unreserved apologies</b>, Alice. We will fully compensate you for the losses incurred.'
• Alice: 'The error on your part has caused considerable damage.'<br>