The vast majority of EU states have agreed to create what some have called the nucleus of a joint army.

Twenty three out of 28 EU states signed the declaration in Brussels on Monday (13 November), prior to making a legally binding pledge at an EU summit next month.

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British firms stand to lose EU defence contracts if UK stays out (Photo: Foreign & Commonwealth Office)

Britain, Denmark, Ireland, Malta, and Portugal stayed out.

But some of them, such as Ireland, indicated they might join in time for the summit. Britain, which is leaving the EU, could also take part under special conditions.

EU foreign relations chief Federica Mogherini called the signing ceremony an "emotional moment".

She said that, from the point of view of her native Italy, the decision had "dismantled the ghosts of the past" and showed that "the taboo concerning EU defence could be broken".

She also said she hoped "this could be an inspiration for other areas of [EU] integration".

Sigmar Gabriel, the German foreign minister, called it "a milestone in European development".

Monday's defence accord is to see participating states jointly develop rapid reaction forces and new materiel such as tanks and drones. It will also see them create single European logistics and medical support hubs.

It involves binding national plans to increase defence spending and military R&D.

It will also be backed up by previous EU decisions to create a single hub for overseas military training missions and a €5.5 billion fund to help member states buy high-end weapons.

The military push, first envisaged by pro-EU politicians on continental Europe in the 1950s, was designed to show European unity in the face of Brexit.

It was also a reaction to an inward turn by the EU's main security ally, the US, under its new president Donald Trump and to heightened instability following Russia's invasion of Ukraine and a string of Islamist terrorist attacks.

The final shape of the project was less ambitious than some EU states, such as France and Italy, had called for.

But Italy had earlier said it would still mark "the initial nucleus of a future European integrated force".

The decision was taken because "the world was changing", Mogherini said.

"It comes at a time of significant tension," French foreign minister Jean-Yves Le Drian said.

The defence declaration comes despite previous concerns about the project by anti-federalist states such as the UK and Poland and by neutral countries such as Ireland.

The UK, which is one of the EU's only nuclear powers (along with France) and its top military spender, is to leave the bloc in early 2019.

But Monday's accord said "third states may exceptionally be invited" to join the EU military club, so long as they provided "substantial added value" and had no "decision powers".

Boris Johnson, the UK foreign minister, said Britain would "support" the scheme. "We are there, like a flying buttress to support the cathedral," he said.

Polish foreign minister Witold Waszczykowski said Poland might opt out in future if the EU "experiment" fell foul of Polish concerns.

He said it should not duplicate Nato and should not lead to Polish defence firms falling into foreign hands.

The Irish foreign minister, Simon Coveney, hinted that Ireland would join in December.

"This does not undermine in any way Irish neutrality", he said, noting that participation in specific projects, such as EU rapid reaction forces, would remain "voluntary" even if Ireland signed up.

Jens Stoltenberg, the Nato head, who attended Monday's EU meeting, said the project was "good for Europe and good for Nato" because it would lead to more spending on defence.

With 22 EU countries also being members of the transatlantic alliance, he said it would "strengthen the European pillar within Nato".

He added that any new EU "forces or capabilities" should "also be available" for use in Nato operations.