A referendum would be "inevitable" if plans to give the UK a written constitution go ahead, a minister says.
The government says it wants to give Parliament more power
Justice minister Michael Wills said any "fundamental alteration in the powers of Parliament" was likely to make a vote by the public necessary.
He spoke after the government said a written constitution could be introduced following wide consultation.
The Tories called Mr Wills' comments "extraordinary" following a vote on the EU treaty being ruled out.
Mr Wills, speaking after the various proposals were outlined in Parliament, said: "These changes are going to be profound... This is going to change fundamentally the way power is distributed in this country."
The government is putting out to consultation a wide range of suggestions - including changing the way judges are appointed and giving Parliament the final say on going to war.
Asked if eventual plans for a written constitution would make a referendum necessary, Mr Wills replied: "I think it would be inevitable."
He said such changes would be "profound", adding: "How fast we drive it depends on the appetite the British people have."
Mr Wills' comments come as the government has said there is no need for a referendum on the European reform treaty.
The Tories say the EU treaty brings with it fundamental constitutional change, but ministers argue that this is not the case.
Mr Brown says that the EU treaty is the latest in a series of amending treaties on which Parliament has always had the final say.
The government had promised a referendum on the now axed EU constitution, on the basis that it would have had implications for the UK's constitution.
Following Mr Wills' comments, shadow justice secretary Nick Herbert said: "This is extraordinary given that the government has spent the last month arguing against giving the people a say over such matters.
"No one will believe a government minister who promises a referendum when they have reneged on a manifesto promise to hold a referendum on the EU constitution."