"Meeting Class Requirements: It’s possible for a character to take levels in a prestige class and later be in a position where the character no longer qualifies to be a member of the class. An alignment change, levels lost because of character death, or the loss of a magic item that granted an important ability are examples of events that can make a character ineligible to advance farther in a prestige class. If a character no longer meets the requirements for a prestige class, he or she loses the benefit of any class features or other special abilities granted by the class. The character retains Hit Dice gained from advancing in the class as well as any improvements to base attack bonus and base save bonuses that the class provided." - Complete Warrior, page 16
It's not just the first level; quite the opposite.