The Foreign Secretary has said Britain is “supportive” of a European Union (EU) deal laying the foundations of an EU army, despite historically opposing it as a challenge to NATO.

This Monday, led by France and Germany, 23 member-states signed up to a common defence fund, which will see them develop and deploy armed forces together.

The Permanent Structured Cooperation (PESCO) pact also includes commitments to integrate armed forces, a boost in defence spending, and the establishment of a joint headquarters.

EU bosses also earmarked £4.9 billion (€5.5 billion) to fund research and development into new military hardware and the joint purchase of equipment.

Pro-Brexit campaigners warned of an EU army before the referendum, and many people voted to leave the bloc to avoid it. Yet, despite the Brexit vote, the UK could still be sucked in.

Countries that are not in the EU can take part in specific PESCO missions and Boris Johnson, the foreign secretary, seem to imply Britain could participate after Brexit.

“The UK is supportive of this,” he said, according to The Times. “We are there like a flying buttress to support the cathedral. We think there is a lot of promise in the ideas.”

The UK Independence Party said they welcomed the government not signing up to PESCO, but urged the government to protect the independence of the British military into the future.

Party Leader, Henry Bolton, said: “Whilst on this occasion the government backed down from giving Brussels authority over some British military assets, there are other agreements that the government has signed which delegate power to EU structures.

“By doing so the Tory government has betrayed our service personnel and compromised our nation’s ability to protect itself independently. So we must now work to ensure that these agreements are not implemented.

“The military intentions of Jean-Claude Juncker are clear: centralised authority and political control over the armed forces and defence expenditure of EU member states. Under no circumstances must the UK to enter such a scheme.

“The UK must preserve its own, full spectrum and sovereign military capabilities free from external control. Whilst we will cooperate with and support our friends and allies to ensure collective security, we must protect the integrity of the NATO alliance and our own ability to defend ourselves.”