Former HM Inspector of Constabulary, Lord Dear, will lead a vote against the Government’s same-sex marriage Bill on 3 June.
If he secures a majority, it will be the end of David Cameron’s controversial Bill to redefine marriage.
Lord Dear has previously defeated the Government in the Lords over free speech and 42-day detention without charge.
In a letter to fellow Peers, Lord Dear said: “the Government is using its raw power to press ahead as fast as possible.
“The political parties have no mandate from the public to bring it forward: the redefinition of marriage was not included in any party manifesto.
“The official consultation process massaged the results to downplay opposition and has been widely dismissed as a sham.
“The House of Commons process has been marked by guillotined debates and undue pressure from party hierarchies on individual MPs.
Lord Dear said the Lords have the authority to vote down a Government Bill “when the executive is clearly acting with no democratic mandate and without regard to the proper constitutional processes.”
Before the Bill reaches the Lords, MPs must wrap up their consideration of the Bill on Monday and Tuesday.
The Government has promised to consider the case for allowing heterosexual couples to enter civil partnerships, further unravelling the place of marriage.
A raft of amendments have been tabled by backbench MPs to protect the civil liberty of people who believe marriage should remain as it is.
And an amendment calling for a referendum on the issue has also been put forward.
The Labour Party is whipping its MPs against the amendments, and it is not yet clear whether Labour MPs will have a free vote at Third Reading.
The Conservatives and the Lib Dems are allowing MPs to vote according to their consciences.