Tony Blair has said he will not sign a treaty at this week's EU summit giving up control of British law to Brussels.
Tony Blair: UK demands mean a referendum will not be needed
He said there were four key areas where he would not compromise: the Charter of Fundamental Rights, foreign policy, common law, and tax and benefits.
He told MPs that his stance meant there would be no need for a referendum on any treaty that emerges from talks.
But the Conservatives say any deal that hands power to the EU must be put to the public in a referendum.
Shadow Europe Minister Mark Francois said: "These are not so much red lines as red herrings. For instance, he said nothing about a new EU president or giving the EU the right to sign treaties.
"Nor did he make clear his views on the crucial details in other areas that could see major transfers of power to the EU."
He added: "What really matters is simple - that the government should stick to its promise and give the British people the final say in a referendum if a new treaty hands over powers from Britain to the EU by bringing back parts of the rejected constitution."
Mr Blair pledged in 2004 to hold a referendum on the draft EU constitution, before it was rejected by voters in France and the Netherlands.
German Chancellor Angela Merkel, who is currently chairman of the EU, is expected to push this week for a new treaty preserving the bulk of the constitution.
She is expected to lobby for the EU to have a "single legal personality", a legally-binding charter of fundamental rights and more majority voting in criminal law and foreign policy.
Europe Minister Geoff Hoon has told the BBC Gordon Brown, who will take over as prime minister at the end of this month, would hold a referendum if significant powers were transferred to the EU at Thursday's summit.
Labour MP Geoffrey Robinson, a close ally of Mr Brown, told BBC Radio 4's The World at One that it would be "absurd" to rule out a referendum "in advance of the negotiation".
But he said he did not think there would be a need for one as the likely deal would not "amount to any significant transfer of powers".
In his final appearance before the Commons liaison committee, Mr Blair said he wanted a deal to "amend" existing EU treaties.
"If people want an agreement this week we have got to go back to a conventional amending treaty," he told MPs.
And he set out the areas where he said he would not compromise at the summit, which will be his final major engagement before stepping down as prime minister.
"First, we will not accept a treaty that allows the charter of fundamental rights to change UK law in any way.
"Second, we will not agree to something that replaces the role of British foreign policy and our foreign minister.
"Thirdly, we will not agree to give up our ability to control our common law and judicial and police system.
"And fourthly, we will not agree to anything that moves to qualified majority voting something that can have a big say in our own tax and benefit system. We must have the right in those circumstances to determine it by unanimity."
He added: "If we achieve those four objectives I defy people to say what it is that is supposed to be so fundamental that could require a referendum."
He said a treaty was needed to make an expanded Europe of 27 nations work effectively but the government could "never satisfy" Eurosceptics who he said wanted to take Europe in a "backwards" direction.
"There are people who will say if there is a comma from the constitutional treaty that goes into the new treaty who will say it is a fundamental matter and has got to be put to a referendum," Mr Blair told MPs.
But he also revealed that he did not believe there had been a need for a referendum on the constitution in its original form.
"I never had the feelings about the constitutional treaty that other people had because we had actually negotiated a very good deal for the UK.
"But I accepted in the end it was a treaty that - in the way it was put forward - led to people to say this is something of such a fundamental nature that it should be put to the British people. I didn't always agree with that."
The UK Independence Party, which campaigns for Britain's exit from the EU, dismissed Mr Blair's assurances as "a decoy to mask what is really going on".
"If there is nothing in the treaty that the British people should object to, then why is he too scared to let us have a say in a free and fair referendum?," said UKIP leader Nigel Farage.
"Mr Blair has outlined what he won't sign up to, but shouldn't he have the decency to tell us what he will agree on?
"Are Mrs Merkel and [French president] Mr Sarkozy really going to be held back from their dreams of a new EU Treaty by a dead duck prime minister who only has days left as leader?" he added.
Mr Blair will step down as prime minister on Wednesday next week.