Full Idea
For Russell there is a logical entailment: 'the x is F' entails 'there exists one and only one x'. Whether or not this is true of the attributive use of definite descriptions, it does not seem true of the referential use. The existence is a presumption.
Gist of Idea
'The x is F' only presumes that x exists; it does not actually entail the existence
Source
Keith Donnellan (Reference and Definite Descriptions [1966], §VI)
Book Reference
'Naming, Necessity, and Natural Kinds', ed/tr. Schwartz,Stephen P. [Cornell 1979], p.54
A Reaction
Can we say 'x does not exist, but x is F'? Strictly, that sounds to me more like a contradiction than a surprising rejection of a presumption. However, 'Father Xmas does not exist, but he has a red coat'.