The UK will never be part of an EU army, the government has insisted.
Following a report that Brussels is moving to promote greater military cooperation between EU member states, 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."
The Times newspaper reported that a policy paper drawn up by the EU foreign policy chief Federica Mogherini suggests the organisation could "step up" its contribution to security defence.
The plans are reportedly being kept secret until after the UK's referendum on EU membership on June 23.
The Global Strategy on Foreign and Security Policy urges the EU to create defence structures using mechanisms set out in the 2009 Lisbon treaty.
The paper can only be read by a select group, who must leave electronic devices outside a sealed room, according to The Times.
The proposals will not be sent to national governments until the day after Britons vote in the EU referendum.
Former defence secretary and Brexit campaigner Liam Fox warned that the proposals would weaken Nato and undermine security in the UK.
"This is our last chance to stop being dragged into a permanent EU military force," he told the paper.
A Britain Stronger in Europe spokesman denied that Britain would ever be part of a European army because such plans could be vetoed.