Proposals include the creation of an EU military HQ, with medical aid and logistics capabilities, possibly in Brussels, that would command EU military and civilian missions.
The UK has voiced opposition to the plans, arguing they would drain away finite resources when Nato already has its military command centre, also in Belgium.
Speaking in Parliament, UK ECR group MEP Geoffrey Van Orden, an acknowledged military expert, branded the move as a "dangerous step" which would undermine Nato.
RELATED CONTENT
The deputy told a meeting of the security and defence subcommittee he fiercely opposed the EU setting up military structures that were independent of the western alliance.
"Let's be clear," he said, "when the UK leaves the bloc, the EU budget is going to be significantly smaller but the EU will still have many demands on the budget in the defence area.
"Do not be taken in by those who argue that a European defence union will be the answer to all Europe's security problems because it won't. Setting up separate structures will only cause divisions," the ECR member stated.
He also questioned how the EU would finance a defence union, adding, "The UK is the second largest net contributor to the EU and when it leaves the EU budget is going to be significantly smaller.
"Instead of talking about new structures, the EU should be making best use of Nato."
However, another committee member, German EPP group deputy Michael Gahler, defended the idea, saying, "If it is going to act in its own interests, the EU needs some sort of military autonomy.
"This is not about leaving Nato or relegating Nato to a secondary level. Not at all and see no contradiction in the creation of a defence union and our continued membership of Nato."
The deputies were taking part in a discussion about a draft report on the defence union drawn up by ALDE group member Urmas Paet.
The debate on Monday comes after British defence Minister Michael Fallon recently said the UK would veto the creation of EU military capabilities so long as it remained a member of the bloc.
The proposals for a defence union have been endorsed by France and Germany, the European Commission and many eastern European countries.
German defence Minister Ursula von der Leyen said recently that she does not expect Britain to block the Franco-German initiative.
She told Reuters in an interview that she expected Britain to "make good its promise that it will not hinder important European Union reforms."
In his recent state of the union speech, Commission President Jean-Claude Juncker said, "For European defence to be strong, the European defence industry needs to innovate. That is why we will propose before the end of the year a European defence fund to turbo boost research and innovation."
The European Union has long considered forging closer defence ties while not undermining the US-led Nato alliance, to which many EU member states also belong, especially in the face of a more aggressive Russia and worsening conflicts in the Middle East.
The decision of the UK, a staunch opponent of any "EU army", to quit the EU has also removed an obstacle to the closer European defence cooperation.