The Deputy Prime Minister is due to convene a round-table seminar with think tanks and democracy campaigners at Westminster.
The Liberal Democrat leader, who entered a historic coalition with the Conservatives in May 2010, will tell the group – expected to include the Electoral Reform Society, Unlock Democracy and 38 Degrees – he is more convinced than ever of the need to reform.
Opening the Portcullis House meeting, Mr Clegg is expected to say: “I have looked at the institutions of our establishment close up, and I can tell you, I am more determined than ever to see them change.
“Britain’s broken establishment is now well past its ‘sell-by date’.”
Mr Clegg will link the parliamentary expenses scandal, the banking crash, party funding rows and the “sordid spectacle” of phone hacking.
The Liberal Democrat leader, who has faced criticism in the wake of declining poll numbers and local election setbacks, is expected to say Britain “is not broken at all”, but will argue: “It is the British establishment that is broken. It is the institutions at the top that have let down the people.
“As political leaders, David Cameron, Ed Miliband and I all have to step up, and make sure we seize this moment.
“I really think if we can’t tackle these deep problems in the established institutions of our country now, you’d have to wonder if we ever can.
“It is now or never. We have to show real courage.”
Mr Clegg will tell the group it is a “critical moment”, saying: “The economic crisis and political crisis aren’t separate. They are part and parcel of a deep failure of our established institutions. You cannot build a healthy economy without strong, clean politics.”