A privately funded referendum on the EU treaty should be held if Gordon Brown refuses to call one, says a Tory peer.
Critics say the treaty is the constitution by another name
Ex-employment secretary Lord Young told the BBC it could be a last resort and would not be short of sponsors.
It follows calls from Labour MP Gisela Stuart, who helped draw up the original constitution, for a referendum.
The government says a referendum is unnecessary as the EU treaty is not the same as the constitution - and the UK has secured a series of opt-outs.
When Tony Blair was prime minister he had promised a referendum on the constitution - which was abandoned after it was rejected by French and Dutch voters in 2005.
The new treaty, expected to be finalised later this year, preserves much of the original constitution - but the government says it does not transfer "in any significant way" any UK sovereignty to the EU.
However Lord Young told BBC Radio 4's Today programme he believed it was the "biggest constitutional change this country's ever gone through" and pressure had to be kept on the government to hold a referendum.
"It's not a Conservative matter, there's a large number of Labour MPs who are now pushing towards having a referendum," he said.
Lord Young said a privately funded referendum was rare but not unprecedented - pointing to Stagecoach tycoon Brian Souter's poll on the repeal of Clause 28 - which prevented local authorities from promoting homosexuality - in 2000. It was said to have cost him £1m.
Lord Young said there were "a whole lot of things" that could be done before getting to that stage, to put pressure on the government to hold a referendum.
"This is the end of the road, I hope long before that Gordon will have changed his mind," said Lord Young.
But he said: "I have no doubt at all it will be possible, at that time, to raise the amount of money that is concerned.
"There are a large number of people who feel extremely strongly about the government breaking its word."
last week Ms Stuart said "all the big items" from the abandoned constitution had been retained and the British people should be asked to endorse the treaty.
Mr Brown has accused the Conservatives of "retreating to the old agenda" on Europe. Foreign Secretary David Miliband has said the treaty "takes forward institutional reform in a sensible way".