Sir Menzies Campbell has called for members of the public chosen at random to be involved in drawing up a written constitution.
Writing in Thursday's Guardian newspaper, the Liberal Democrat leader stated that "British politics is in crisis" and should not be left to politicians alone to fix.
Sir Menzies is set to put to his party conference later this month plans for fixed-term parliaments, the appointment of departmental secretaries of state to be subject to scrutiny by select committees and a rule that no new prime minister should be allowed to take office before presenting their programme for government to parliament.
However it is the call for jury trial-style plans for public involvement that are likely to cause the most controversy.
"If British democracy is not to slip into terminal decline, the public must be alerted to the dangers and motivated to participate in its radical reform," he argued.
"Revival of our representative democracy is too serious to be left to politicians."
"As a first step, a written constitution is essential to ensure that the sovereignty of the citizen is established," Sir Menzies added.
"I don't believe that the rules of the political game can gain or retain public trust if they are drawn up by politicians alone.
"That's why we propose a constitutional convention, at least 50 per cent of which would be composed of members of the public, drawn by lot. This will put the political process on a real jury trial."
Dods Parliamentary Communications Ltd