Reality Check: Will the UK end up joining an EU army?
The Claim: Veterans for Britain says the EU is planning the creation of an EU army, which would mean the UK would lose control of its defences.
Reality Check verdict: Many European politicians want to see the creation of an EU army, but EU treaties are clear that it could be blocked by Britain.
A group of retired, senior military officers has issued a warning that staying in the EU would damage Britain's military effectiveness. They say that EU law interferes with operational freedom and that moves to create an EU army are undermining Nato.
But could the UK be forced to join a combined army if it stayed in the EU?
At the moment, Britain takes part in EU military operations through the Common Security and Defence Policy, but those are focused on peace-keeping and humanitarian assistance. And Britain has previously argued against proposals to integrate further the armed forces of member states.
However, there are many senior European politicians who would like to see deeper integration. Jean-Claude Juncker, the EU Commission president, has said that a European army is needed to stand up to Russia. And a recent German government white paper called for progress towards an EU army.
That could be blocked though. Article 42 of the Lisbon Treaty states that what it calls "a common defence" would require unanimity in the European Council. In other words, Britain could veto it. It goes on to say that any such decision would be taken in accordance with the constitutional requirements of member states. That would mean ratification in national parliaments and it could trigger the UK's European Union Act 2011, which would mean a referendum before any powers could be transferred.
Steve Peers, professor of law at the University of Essex, says it is "clearly incorrect" to say that the UK could be forced to join an EU army.
Furthermore, the treaty is clear that any further co-operation in the area of defence should be consistent with Nato commitments.
However, other countries could decide to work together more closely on defence even if the UK wasn't involved. And there will probably be pressure from other countries for more co-operation, even if that falls short of an EU army or any formal transfer of powers.