Prime Minister David Cameron is ready to block any attempt to give the EU a bigger role in co-ordinating defence policy, his office has indicated.
He is attending a summit of EU leaders in Brussels, the first to discuss defence for five years.
The UK wants to emphasise the primacy of Nato and nation states, although Nato is keen for European countries to contribute more to their own defence.
Downing Street fears any move by the European Union could undermine Nato,
European officials have privately questioned the UK's stance.
The UK has worked to include more references to Nato in the summit's conclusions and Mr Cameron has made it clear he would block the plans if necessary.
British officials have also been pointing out that defence is a matter for member states and they do not want to see the EU intruding into these areas.
They also objected to references to drones.
Responding to the UK government comments, a European Commission spokesman said the Commission had "no intention to own or procure its own drones".
Nato chief Anders Fogh Rasmussen is attending the talks, at the request of Mr Cameron.
At a Brussels news conference Mr Rasmussen said "we're not talking about the EU possessing capabilities; there is a need for nations to do more to acquire much-needed military capabilities".
He said military operations had shown a need for more European observation drones, air-to-air refuelling planes and heavy transport planes.
A Downing Street source said there could be no question of the European Commission owning military capabilities such as drones.
The source said: "The prime minister will be making clear the primacy of Nato.
"We see Nato as the bedrock of our collective defence. Any EU action should be complementary to that, but not duplicating it.
"We should be very clear that defence is a member state power and we don't want to see an extension of EU action in this area.
"What the EU does do should be focused on practical action, facilitating what member states may want or choose to do together.
"Take drones as an example; there can be no question of the commission owning dual-use military capabilities such as drones."
Mr Cameron is also expected to stress that the role of the EU's Common Security and Defence Policy, set up four years ago, should be in support of member state action.
BBC Europe editor Gavin Hewitt said European officials privately questioned why the UK had insisted on these changes at the last moment when they have had the conclusions for two weeks.
When they meet on Thursday and Friday, EU leaders are due to discuss how to strengthen Europe's defence industry and hone its common defence policy.
BBC defence correspondent Caroline Wyatt said the US - Europe's traditional guarantor of security - had begun to shift its strategic focus to Asia and the Pacific.
This was putting pressure on Europe to strengthen its own defence capabilities on the continent and in neighbouring North Africa - areas where France and Britain have already worked together, for example in the Libya campaign.
Mr Cameron will use the summit to repeat his warnings about opening borders to immigrants from Romania and Bulgaria, which will happen from 1 January.
The Downing Street source said: "You can expect the prime minister to use it as an opportunity to reiterate his concerns" said the Downing Street source.