The UK will "never be part of an EU army", the Government said as reports suggested moves in Brussels to promote closer military co-operation among the bloc's member states.
A policy paper drawn up by the European Union's foreign policy chief suggests the organisation could "step up" its contribution to security defence, The Times reported.
Former defence secretary and Brexit campaigner Liam Fox warned the proposals would weaken Nato and undermine security in the UK.
But a Government spokesman said: "The Prime Minister has repeatedly made clear that the UK will never be a part of an EU army. We retain a veto on all defence matters in the EU and we will oppose any measures which would undermine member states' military forces."
According to the newspaper, the plans being prepared by Federica Mogherini are being kept secret until after the UK's referendum on EU membership on June 23.
The plans will not be sent to national governments until the day after Britons vote and until then the Global Strategy on Foreign and Security Policy can be read by only a select group, who must leave electronic devices outside a sealed room, The Times reported.
The newspaper has seen handwritten notes about the document, which emphasises that "security and defence is where a step change is most urgent" in terms of how the EU functions.
The paper urges the EU to create defence structures using mechanisms set out in the 2009 Lisbon treaty.
"The EU can step up its contribution to Europe's security and defence," the draft said.
"Our external action must become more joined up across policy areas, institutions and member states. Greater unity of purpose is needed across the policy areas making up our external action."
The proposals refer to "permanent structured defence co-operation" powers that allow a group of nine or more EU states to press ahead with plans for a military headquarters.
Fox told the newspaper the plans would "weaken Nato and undermine our security to boost the EU's dream and continued aim of ever closer union".
He said: "This is our last chance to stop being dragged into a permanent EU military force."
A Britain Stronger in Europe spokesman said: "As part of the EU, Britain will never be part of a European army because we can veto it.
"If we leave the EU, they could create a European army as we would not be at the table to veto it.
"The former heads of Nato, MI5 and MI6 have all said our country's national security is threatened if we leave. That's why we're safer remaining in Europe."