Tony Blair has said the UK will not allow the new European constitution to diminish British sovereignty over the "red lines" of tax, defence and foreign policy.
Blair says British agreement is needed for the treaty
Speaking during a break in talks with fellow EU leaders in Brussels, the prime minister again insisted he would not accede to demands for a referendum on the draft constitution.
Earlier Foreign Secretary Jack Straw insisted there was "no case" for a vote on the issue.
Mr Blair said: "If this constitution were to mean the end of us as a nation state then it would be a different matter but it doesn't.
"What's more, take it from me there is nothing we are going to agree to here that's going to put at risk any of these key red lines that we have set out."
The prime minister described reports that the constitution could challenge the Queen's position as "hairs and scares".
He later issued a statement saying specifically: "There will not be a referendum."
Amid reports of US concerns, the prime minister reiterated his commitment to Nato, saying he would never put the alliance at risk.
The UK could be simultaneously strong in its commitment to Europe and retain its close ties with the US, argued Mr Blair.
He said he thought everyone would accept in the end that defence guarantees should be rooted in Nato.
"There is nothing that can remain in any treaty without British agreement since it requires unanimity," he said.
French President Jacques Chirac acknowledged the defence differences as he pressed for a European defence scheme that was totally open to all but also compatible with Nato.
President Chirac told journalists: "The idea of European defence without the United Kingdom, it's important to recognise, is not very coherent.
"It does cause problems for our British friends for obvious reasons. Yesterday, we talked frankly about those problems with our counterparts and we are continuing our discussions."
Mr Straw told BBC Radio 4's Today programme he was not happy with elements of the draft constitution relating to defence.
"We want to see changes to the draft articles on the convention of the future
of Europe because they are unsatisfactory and they don't sufficiently clearly
specify the pre-eminence of Nato as the alliance on which Europe as well as
North America depends for its territorial defence," he said.
"We want to make sure that in Article 140 the pre-eminence of Nato is very
clearly established - it is ambiguous at the moment - but also where there is
so-called constructive co-operation that operates on an inclusive not an
Twenty five European leaders are having a second day of meetings in Brussels to try to resolve differences over the proposed EU constitution.
During the first day of talks, the prime minister's official spokesman said it was a "reasonable representation" of the government's view to suggest that, if certain red lines on vital issues are breached, a referendum might be an option.
But he stressed the prime minister did not expect to be defeated on the key issues of concern.
Mr Straw: EU defence cooperation will not undermine Nato
Conservative shadow foreign secretary Michael Ancram said the prime minister had "slapped down" his own spokesman with his "arrogant" statement.
"This is yet another smokescreen from Tony Blair," he said. "Again he is trying to
hoodwink the British people. Blair's red lines are red herrings."
Liberal Democrat foreign affairs spokesman Menzies Campbell said: "It is
difficult to understand why the Government is so implacably set against a
"What have they to fear if they have confidence in their case?"
The summit follows the publication of an article by Chancellor Gordon Brown who said the idea that the EU should harmonise taxes and become a federal state was "out-dated" and at odds with most people's thinking.