EU Plus - Treaty outside EU could be illegal opines Der Spiegel

The article in Der Spiegel, linked here, is pretty explicit on the potential illegality of extensive use of EU mechanisms outside the remit of the present treaties:

The euro-zone 17 in combination with six other countries quickly began moving forward on their own. But is such a move legal? European Union lawyers have their doubts that the kind of euro-zone fiscal union within the EU would be allowed.

Changes to the EU treaty, after all, must be unanimous. Furthermore, EU officials in Brussels say, because monetary union is regulated extensively in the Lisbon Treaty, reform can only be implemented within the existing legal framework. The legal services experts of the European Commission, the European Central Bank and the European Council, which represents the member states in Brussels, are all in agreement. A treaty concluded only by the 17 euro-zone governments would be illegal, they say.

Individual countries could only issue a "political declaration of intent," in which they determined, for example, how they would decide on the use of sanctions against budget offenders. But such a declaration would have no legally binding character and, as officials point out, could also be revoked following the election of a new government. This is principally a reference to France, where the Socialist presidential candidate François Hollande has already announced that he would not accept any incursions into national sovereignty.

More from the same source is linked here.