Whereas Ireland, Portugal and Malta have not yet decided whether to join the initiative, Denmark and the United Kingdom are not expected to participate.
The EU will move towards closer defence ties on Monday with more than 20 states signing a landmark pact that aims to boost cooperation after Brexit and counteract Russian pressure.
"Today we will launch a new page for the European Defence", said Frederica Mogherini, the EU's foreign and defence policy representative.
"This is the beginning of a common work - 23 member states engaging both on capabilities and on operational steps, that's something big", Mogherini said.
European Union member states will be able to develop military capabilities, invest in joint projects and increase the readiness of their troops.
"We are agreeing on the future cooperation on security and defence issues ... it's really a milestone in European development", he added.
Countries that are not in the European Union could take part in specific missions, opening the way to possible participation by Britain after Brexit - though they will have no role in decision-making.
Those who didn't sign initially can still join at a later date and countries not living up to their expected commitments could be kicked out of the group.
There are strong indications that British officials are pushing hard for the United Kingdom to be included in the Permanent Structured Cooperation process, or PESCO, which is key to the Defence Union plans set out by President of the European Commission Jean-Claude Juncker recently.
The notification letter said PESCO "shall be consistent with commitments" under North Atlantic Treaty Organisation. "The real problem is not how much we spend, it is the fact that we spend in a fragmented manner". The EU, she said, has tools to fight hybrid warfare - the use of conventional weapons mixed with things like propaganda and cyber-attacks - that the military alliance does not have at its disposal.
The notification letter - co-authored by France, Germany, Italy, and Spain - described the pact as "an ambitious, binding, and inclusive European legal framework for investment in the security and defense of the EU's territory and its citizens".
Gabriel said working together is "more economical than if everyone does the same".