Tony Benn has made an impassioned plea to Gordon Brown to give the British people a referendum on the EU treaty.
Mr Benn said he was "terrified" that Germany has not had a referendum
The veteran left-winger said it was the "most bureaucratic, terrifying system in the world and it's being imposed on us on the grounds it's tidying-up".
"If tidying up involves tearing up the British constitution, it's a very interesting definition," he added.
Mr Benn told a Labour conference fringe meeting he thought Mr Brown would eventually cave in to calls for a vote.
"I think we have a good chance of winning this because the public are with us and you cannot deny the public forever", said the former Labour MP.
The prime minister has repeatedly rejected calls for a referendum on the EU treaty, arguing it is not the same as the constitution rejected by voters in France and Holland, on which a vote was promised in Labour's 2005 manifesto.
Earlier this month, the trade unions joined calls by the Conservatives and UKIP for a referendum.
Two national newspapers have also launched campaigns for a vote, with The Sun earlier devoting its first four pages to the subject, claiming Mr Brown would gain a 17% advantage at the polls if he agreed to hold one.
Speaking at a Labour Euro-safeguards campaign fringe meeting, Mr Benn said he did not want to retreat into the "old nationalism" as it led to too much bloodshed.
But he said the "absolutely undemocratic" EU should be reformed and power handed back to nation states and elected politicians.
"Even if I believed in it, I think this system will end up with the break-up of the European Union and I do not want that to happen either," said Mr Benn.
"It terrifies me that Germany did not allow its own people to have a referendum on whether Germany should enter the European Union and have a treaty.
"And, you know, before the war, Hitler wasn't exactly a democrat. And that practice of everything being decided at the top has been carried on.
"France had a referendum, defeated it, and now Sarkozy is trying to bypass it. We live in a continent where increasingly power has gone to a group of people who are not elected, cannot be removed and don't have to listen to us.
"I mean [European trade commissioner Peter] Mandelson is a very powerful man. He wasn't even elected by the House of Commons."
'Death of democracy'
Mr Benn warned: "If this goes through Gordon Brown will not be the prime minister of Great Britain, he will be the mayor of the Greater British Authority".
He said power should be restored to nation states to opt out of the treaty if their people wanted it.
He said he feared the "death of democracy" in a wider sense - with the increasing dominance of unelected bodies such as the World Trade Organisation and multinational corporations.
Even Labour's own conference, which has voted to end emergency motions proposed by delegates and trade unions, had stifled democracy, he added.