The Liberal Democrats are calling for a referendum on Britain's membership of the European Union.
Sir Menzies refused to back calls for an EU treaty referendum
Party leader Sir Menzies Campbell earlier this week refused to back calls for a referendum on the EU treaty.
But he said the public deserved an "honest debate" on Europe - and "that means a referendum on Britain's membership of the EU".
His call comes as the Lib Dems head for their annual conference, which gets under way on Sunday.
Sir Menzies told BBC News 24: "I want to argue the case for Europe. I am a pro-European.
"That case has gone by default too often in the past 10 years of this Labour government".
He said he wanted an "out and out debate" to flush out Eurosceptics on the Conservative benches who had "taken comfort" in that party's call for a vote on the EU treaty.
But shadow foreign secretary William Hague, for the Conservatives, said Sir Menzies was trying to cover up divisions on Europe in his own party.
"This proposal is a clear sign of desperation from Ming Campbell, whose party is so split on this issue.
"The Lib Dems promised a referendum on the EU Constitution.
"They should concentrate on making sure that this promise is kept, instead of producing ill-thought out distractions on the eve of a difficult party conference."
Prime Minister Gordon Brown has repeatedly ruled out a referendum on the EU treaty, despite calls from the Conservatives, trade unionists and a growing number of his own MPs.
Mr Brown argues a referendum is not needed as the treaty is very different to the failed EU constitution, on which a referendum had been promised in Labour's 2005 manifesto.
Sir Menzies said he agrees with the prime minister - but as an ardent pro-European he is "not prepared to allow [Conservative leader] David Cameron to lead the Europhobes and their allies in sections of the media, to distort the debate on Europe without challenge".
The Lib Dem leader, who was expected to face calls to back a treaty vote from some of his own MPs at his party's conference, said voters should be given a "real choice".
"If there is to be a referendum it shouldn't be restricted to a comparatively minor treaty. It must be a decision about the EU as a whole.
"Let's have an honest debate on the European Union followed by a real choice for the British people. That means a referendum on Britain's membership of the EU.
"We would ask the British people the big question - whether to remain in the European Union or not.
"I will lead the Liberal Democrats at the forefront of that debate.
"We will make the overwhelming case for Europe and trust the people to make the right choice."
In 1975, Voters backed the UK's continued membership of the European Economic Community (EEC), as it was then called, by a large majority, in the country's first nationwide referendum.
Since then, what was originally an economic agreement has been transformed into a political union through a series of treaties.
Critics, including the UK Independence Party (UKIP), have long argued that the public should be given another say over Britain's continued membership.
But Sir Menzies' intervention is the first time one of the main pro-European parties in Britain has added its voice to such calls.
UKIP welcomes move
UKIP leader Nigel Farage welcomed Sir Menzies' call for an "honest debate" on British membership of the EU.
"I have believed for some time that the only referendum that Gordon Brown will ever consider would be one with the new Constitutional Treaty as continued part of our EU membership.
"Brown believes that this is the only referendum on the EU that he can win.
"The parliamentary arithmetic means that if the Lib Dems support such a move it would give the British people their first chance in over 30 years to determine their own futures.
"I would be interested to hear which side in such a referendum Mr Cameron would support."