The UK has worked with other nations on exercises such as Griffin Strike, in 2016 (Picture: MOD).
The European Union is "out of its depth" when it comes to its defence ambitions, even with British participation, a think tank has warned.
The European Union currently has a Common Defence and Security Policy, which sets out its ambitions to participate in peacekeeping and humanitarian missions across the globe.
The International Institute for Strategic Studies (IISS) and the German Council on Foreign Relations has concluded the EU faces "extensive capability gaps".
“Removing the UK from the picture renders a bad situation much worse,” the report adds.
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The one area in which the authors believe the EU is performing without capability shortfalls is in rescue and evacuation operations and support for humanitarian-assistance operations.
However, without the Royal Navy’s contribution even this, they write, is put at risk.
Concern is also raised that ageing equipment is not being replaced.
Whilst support for a European Army has received strong support from France’s President Macron and other EU politicians, the British Government remains opposed.
Defence Secretary Gavin Williamson has dismissed suggestions that the UK could join a European army, and told the House of Commons on Monday:
"Let's make it absolutely clear: Britain is not going to be participating in a European army.”
However, speaking later during a visit to Edinburgh, he made clear that whilst Britain had no plan to join an EU Army, the British Government would remain invested in the continent's security: "Britain has always played a major role in terms of defending not just continental Europe but bringing peace and stability right around the world.
"The cornerstone of defence for Europe and the North Atlantic is not the European Union, it's NATO that's delivered security in Europe for the last 69 years and that is something that Britain leads in.
"We are going to be continuing to invest more and more in defence over the next few years and this is something I think that everyone recognises is really important," he added.
US President Donald Trump has also criticised the proposed European Army, sending tweets aimed at French President Emmanuel Macron top voice his feelings.