Any answer is somewhat speculative, because there are no significant legal precedents.
That said, you are probably not in breach of counterfeiting laws, as they typically protect physical currencies. However, due to the way that laws are written, their scope may be somewhat fuzzy in areas that were not foreseen, so you may find that a law unintentionally does cover your act.
However, even if the act would fall under the law as written, jurisdiction remains an issue. Where would this counterfeiting happen, legally? Again, as this was not foreseen by lawmakers, it's possible that you may fall under foreign jurisdictions, e.g. because you spend the fake BitCoin and the the recipient lives in a country that assumes jurisdiction because their citizen was defrauded.