The EU Treaty will strengthen, not undermine British foreign policy, the UK's foreign secretary has said.
The Tories want a referendum on the Lisbon Treaty
David Miliband said the EU's role would complement, not rival Nato and denied an EU figure would replace the UK on the UN Security Council.
The Tories said the "creeping powers" on foreign and defence policy would erode the UK's ability "to be masters of our own destiny".
EU parliaments must ratify the treaty - signed by EU leaders last year.
MPs debated implications for foreign, security and defence issues on the fifth day of topic-by-topic debates on the Lisbon Treaty, drawn up to replace the failed EU Constitution.
Meanwhile in Strasbourg the European Parliament also debated, and overwhelmingly backed, the treaty on Wednesday.
Opening the Commons debate, Mr Miliband said the decision to create a new EU High Representative for foreign and security issues would give Europe "a more coherent voice internationally".
He insisted the EU would not act "as an alternative to UK foreign policy but as one means for its implementation".
The UK's seat on the UN Security Council would not be affected as only sovereign states could hold a seat.
And on defence policy, provisions in the treaty would help get other EU states to build up forces to "shoulder more of the international burden".
"In short, Britain will continue to decide on its own foreign policy and where we agree with others there can be a common European role in helping to deliver it," he said.
But shadow foreign secretary William Hague said the government itself had fought against many of the defence policy provisions now in the treaty.
He said Jack Straw, when he was foreign secretary, had said it was "simply unacceptable" for proposals made by the EU foreign minister to be agreed by qualified majority voting.
And shadow defence secretary Liam Fox raised concerns that the treaty would undermine Nato.
The "creeping" powers of the EU in foreign and defence policy, he said, "gradually erode our ability to be masters of our own destiny" and would "drive a wedge" between Britain and its transatlantic allies.
"Britain cannot have two best friends when it comes to defence. This treaty asks us to make a choice," he said.
"The Conservative Party will not weaken our transatlantic bonds. We want the EU to work in partnership with Nato, not compete with Nato."
But Edward Davey, for the Liberal Democrats, said his party broadly welcomed the provisions.
He added: "Despite the hysteria being whipped up by some, the changes wrought by the treaty involve no new powers for Brussels but a simple and sensible reallocation of powers between those responsible for this area of policy.
"Foreign and security policy remains, as it always has been, in the control of member states. Britain controls its veto on all key decisions."
And Labour's former Europe minister Denis MacShane accused the Conservatives of having "the most rejectionist, isolationist position on partnership in Europe ever seen by any party in the history of this country".
The debate on the Lisbon Treaty and the European Union (Amendment) Bill which will write it into UK law was adjourned until Monday, when MPs will consider its impact on international development matters.