Apart from Britain, Denmark, Portugal, Ireland and Malta have yet to decide whether to join the pact.
Long blocked by Britain, which opposed the creation of an European Union army, the defense integration drive was revived by France after the election of pro-European Emmanuel Macron as president, and Germany after Britons voted in June 2016 to leave the EU.
According to a statement released by Mogherini's foreign affairs council, the pact on the Permanent Structured Cooperation (PESCO) foresees the possibility of a number of European Union member states working more closely together in the area of security and defense.
"In a nutshell, we have had six EU Council meetings over the past year where United Kingdom officials have agreed various aspects of the Defence Union", he told reporters.
By working together on joint projects, nations hope to use their combined spending power to overcome capability gaps, jointly buying equipment like air transporters or drones.
PESCO will not compete with North Atlantic Treaty Organisation, as the latter specialises in collective defence, whereas bolstered defence cooperation among Europeans is created to ensure that the EU can respond to various situations overseas in a more prompt and efficient manner, claim European diplomats.
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. "The real problem is not how much we spend, it is the fact that we spend in a fragmented manner".
Attendees pose for group photos after signing a pact on joint defense cooperation at the headquarters of the European Council in Brussels, Belgium, Nov. 13, 2017.
"So, today we will launch a new page for the European Defense", said Mogherini. This could take place at the next Foreign Affairs Council on 11th December.
U.S. President Donald Trump refused last month to certify that Iran is complying with its obligations under the nuclear accord, though worldwide inspectors said it is.
Participants have signed up to a list of commitments which "include increasing the share of expenditure allocated to defense research and technology with a view to nearing the 2 percent of total defense spending" and to "regularly increasing defense budgets in real terms".