Britain will not sign up to the EU reform treaty if its "red lines" are not kept, Gordon Brown has said.
The government says a referendum on the treaty is not needed as it has secured control over human and social rights, foreign policy, tax and benefits.
"If we do not achieve our red lines, then I would veto the treaty," the PM told BBC One's The Andrew Marr Show.
The Conservatives, UKIP and some Labour MPs say, regardless of the "red lines", a referendum is needed.
Constitutional change
The reforming treaty incorporates some of the old EU Constitution - which was dropped after being rejected in referendums by French and Dutch voters - on which the Labour government had promised a referendum.
The Conservatives argue it is as much as 90% the same - the government says it has secured opt-outs which make it substantially different.
"We simply do not believe this is not the EU Constitution in disguise"
Mr Brown told the BBC that had it been a decision on the previously proposed constitution or joining the Euro, then he would back a referendum.
But he said if the red lines were achieved, he could not "honestly" say there would be any constitutional change.
He added: "I have got now to make sure that these red lines are implemented in practice.
"I've got to go to Brussels both in October and December and ensure that in detail we have achieved our aim, which is to remove the prospect that there are fundamental constitutional changes as a result of this amending treaty and so we've got a protocol, we've got an opt in, we've got an emergency break in some areas.
"If I can show that then let Parliament and let the country be a judge on that but let them see if I have achieved my aims and then we can have a further debate on it."
Manifesto promise
But UK Independence Party leader Nigel Farage said the opt outs were "a fraud" and one of the "opt outs" - on justice and home affairs, would only last five years.
He said: "They [the government] will do whatever they can to force through their beloved EU Constitution, and aren't going to let a trifling matter like a manifesto promise stop them.
"But we must all send a message to Mr Brown telling him we know what his plans are and we simply do not believe this is not the EU Constitution in disguise, which will fundamentally change the way we are governed."
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