This was made in a 'Note Verbale' a form of unsigned diplomatic memo and supplemented by a written statement in the House of Commons by Amanda Solloway (Parliamentary Under Secretary of State, Minister for Science, Research and Innovation ).
The statement is reproduced hereon the UPC site and explains that its purpose was to "ensure clarity regarding the United Kingdom's status in respect of the Agreements and to facilitate their orderly entry into force for other States without the participation of the United Kingdom". While the stated intention is to clarify and facilitate, it has precipitated a rush of comments from legal experts and UPC-watchers suggesting that the situation is anything but clear. The UPC Agreement has no provisions for withdrawal and even less so for withdrawal prior to entry into force. The Vienna Convention on the Law of Treaties (which should apparently govern such situations) is sufficiently ambiguous to create many blogs worth of alternative interpretations while conspiracy theorists may suggest that the use of a Note Verbale is expressly intended to open a wormhole in the law allowing the UPC to enter into force at warp-speed.
The truth of the matter is that we surely do not know exactly what this means or which is the right path to get out of the present maze. Nevertheless, what is clear is that many players are still passionate enough about the project to keep it afloat. And an idiom that surely has an equivalent in all 24 EU languages is that 'where there's a will, there's a way'.