Lord Irvine of Lairg insisted during the resumed committee stage of the House of Lords Bill that the legislation in its present form was "water- tight" and in "no way unambiguous or obscure in its intent".
The Tories intend to exploit an apparent legal loophole, saying a key part of the Bill is flawed and cannot be implemented until the end of the current five-year Parliament.
Lord Mackay of Ardbrecknish, for the Opposition, quoted a 1632 case when the Earl of Bristol did not receive the customary writ from Charles I but petitioned the House successfully and took his seat.
This meant hereditaries who did not receive the writ would be able to petition the new Parliament for the right to be summoned. "The lawyer says: `Nobody is a member of the House of Lords by virtue of an hereditary peerage. Membership of the House is conferred by obedience to a writ of summons'.
"There are those who say the writ is merely a summons. That once it has been answered, once the peer has come to this House then the writ cannot be used as a means to eject a peer"A peer who has answered the writ and taken his oath, sits here as of right. This Act is going to set aside centuries of constitutional practice and precedent."
But Lord Irvine insisted that the abolition of hereditaries was a "concept which the ordinary man in the street, and the judges, will have no difficulty in construing".
Earlier, Lord Trefgarne, a former defence minister, said peers were determined to defy the Government and petition the Lords on the grounds of the historical precedent once the legislation has passed.
"I think the position will be that if we are excluding from the House at the end of this session by the alleged effect of the present Bill, then come the next session we can come back and say, `Excuse me, we are ready to start again'."