Do you have to wait for confirmation from the journal to consider an article withdrawn? If you'd like to withdraw an article for any reason, can you start submitting elsewhere once you've notified the journal, or do you have to wait for a response? How can this be handled in case the journal is not responsive?
A journal publication is based on a two-sided contract. To be valid (and executable), therefore, both parties need to agree about the key performance of this contractual relationship, i.e., the Author granting to the Publisher an (exclusive) license &quot;to use, publish, edit, reproduce, distribute, publicly perform, publicly display, and prepare derivative works based upon the Work&quot; (to cite § 3.3.2 of the &quot;Model Publishing Contract for Digital Scholarship&quot;).
If, in the negotiation stage (e.g. during peer review), one of the parties notifies to the other that there is no will to feel bound to such a contract anymore, then there is no contract. The publisher cannot claim to have won a &quot;unilateral contract&quot; against the author's will.
As a result, I would say that you are free to submit your manuscript elsewhere as soon as you notify the first journal about it.
(However, note that I am not aware of a court case about this; it is simply my legal interpretation.)