Former trade union chief Bill Morris and ex-CBI leader Digby Jones have clashed over plans to allow unions to expel members because of their political views.
Lord Morris of Handsworth, the former general secretary of the Transport and General Workers' Union, called the proposal "barking mad".
He was speaking in the Lords, in response to plans set out by Lord Jones of Birmingham, the former director general of the CBI.
Lord Jones is now a Trade Minister in Gordon Brown's government while Lord Morris, who spent most of his childhood in Birmingham, became a Labour peer two years ago. But they clashed over Government proposals to give trade unions the right to kick out members if they belong to an unacceptable political party.
It follows a recent European Court of Human Rights ruling which endorsed the right of rail union Aslef to expel members of the far right British National Party.
Lord Jones, speaking for the Government, confirmed in the Lords that new laws would allow unions to ban members for belonging to other parties if they chose, including the Conservative Party.
However, he insisted the Government was merely changing British law to reflect existing European legislation.
Lord Morris warned: "To be judged by our actions - on what we do, not on what we are members of - has long been a cherished tradition of freedom and liberty in our country.
"I ask myself where unacceptable membership starts and where it ends. What organisations will be proscribed? Who will be the judge? Where are the safeguards?
"Is this the first step not only towards expelling someone you do not find favour with but to banning the organisation to which they belong?"
Referring to clause 17 in the Employment Bill, which contains the controversial change in the law, he said: "I ask the good people of Barking to forgive me, but I believe that clause 17 is barking mad."
Lord Jones said other organisations were already allowed to expel members because of their political views and the proposed legislation simply gave trade unions the same rights.
But concerns were also raised by another Birmingham peer, Lord Fowler of Sutton Coldfield, a former journalist who became a Conservative MP and Minister under Margaret Thatcher.
He said: "I am a lifelong member of the National Union of Journalists. I retained that membership when I was a Minister; I continued to pay my subscription, although the National Union of Journalists was not always in agreement with government policy.
"Under the Bill, would it mean that, if it wished, the National Union of Journalists would be entitled to expel me from membership?"
Lord Jones confirmed the union would have the right to expel him for being a Tory.
Government spokesman Lord Bach said organisations such as the CBI and the National Trust already had such powers.