The New Yorker of the 4th December, 1948, quoted a question asked of The Philadelphia Bulletin:
My class would appreciate a discussion of the wrong use of which in sentences like "He wrecked the car which was due to his carelessness",
and the answer given by that newspaper:
The fault lies in using which to refer to the statement "He wrecked the car". When which follows a noun it refers to that noun as its antecedent. Therefore in the foregoing sentence it is stated that the car was due to his carelessness, which is nonsense.
What is? Carelessness? is the New Yorker's query.
Which shows how dangerous it is to dogmatise about the use of which with an antecedent consisting not of a single word but of a phrase.
The fact is that this is a common and convenient usage, but needs to be handled discreetly to avoid ambiguity or awkwardness.
The required statement is in course of preparation and will be forwarded as soon as official records are complete, which will be in about a week's time.
Here it is unnecessary; the sentence can be improved by omitting the words "which will be", and so getting rid of the relative altogether.
The long delay may make it inevitable for the authorities to consider placing the order elsewhere which can only be in the United States which is a step we should be anxious to avoid.
Here the writer has succeeded in using which in this way twice in a single sentence, and shown how awkward its effect can be. He might have put a full-stop after elsewhere and continued "That can only be in the United States and is a step we should be anxious to avoid".