Peers who break the law should be kicked out of parliament, says Nick Clegg.
In a speech on Tuesday evening, the Liberal Democrat leader was set to call for new anti-sleaze rules in the upper House.
His comments come as four Labour peers face investigation for claims they offered to influence government legislation in return for cash.
Lord Truscott, Lord Moonie, Lord Taylor of Blackburn and Lord Snape have all rejected claims of improper activities.
Baroness Royall, leader of the Lords, has said that in addition to the investigation into specific allegations, the chairman of the privileges committee, Lord Brabazon of Tara, is to conduct a wider review of punishments.
The news has led to calls for expulsion or suspension for those who break the rules.
Under existing regulations, peers who breach the code of conduct can only be ordered to apologise.
In his speech, Clegg was set to argue that the latest allegations "highlight everything that is wrong with our political system".
"A system that too often operates on the myth that tradition should somehow always be trusted," he will say.
"The allegations of accepting payment in return for changing legislation are serious enough. But on top of that, the sanctions applied to the peers if they are found guilty are so weak.
"This case exposes the extraordinary protection enjoyed by the political class. One rule for lawmakers and another for everyone else.
"The most the accused peers will have to do if the Lords' investigation finds them guilty is to apologise.
"That is why my colleague Chris Huhne was right to write to the police to establish whether these allegations might carry more serious sanctions under the law.
"That is why I am also now calling on the government to introduce urgent new legislation to change the rules so that peers found guilty of wrongdoing are expelled from parliament.
"It is wholly out of step with the public's legitimate demand for transparency in politics that just because someone happens to be in the House of Lords they are exempt from the most basic standards of accountability."
Dods Parliamentary Communications Ltd