Angela Rayner has said the Labour Party leader should be stripped of the power to appoint people to the House of Lords.
Jeremy Corbyn has controversially nominated former Commons Speaker John Bercow, who faces bullying allegations, for a peerage.
The outgoing Labour leader has also decided to hand his powerful chief of staff, Karie Murphy, a place in the Lords.
Murphy has been criticised over concerns about Labour’s handling of alleged anti-Semitism within its ranks.
Speaking at a deputy leadership hustings in Nottingham on Sunday, Rayner said the Lords should be abolished.
She added: “In the meantime, our leader should not be appointing people into the Lords.
“Our democratic structures here should be putting people into the Lords.”
Rayner, a close friend of leadership candidate Rebecca Long-Bailey, is currently seen as the clear frontrunner in the contest to replace Tom Watson.
Following the hustings, Rayner said: “This is how Labour demonstrates who we are - through deeds not words. We might not be able to scrap unelected peers yet – we need to be in government for that - but we can practice what we preach by ensuring our members and affiliates have a say in who makes law on our behalf.
“People’s peerages would end the Westminster bubble grip on who sits in our second chamber, and put real people in the driving seat.”
The other candidates in the deputy contest are Richard Burgon, Dawn Butler, Rosena Allin-Khan and Ian Murray.
Keir Starmer, the shadow Brexit secretary, has pulled ahead in the leadership contest and was the first candidate to make it onto the final ballot.
But he faces a tough challenge from shadow business secretary Long-Bailey, who has the backing of the current leadership.
Backbencher Lisa Nandy and shadow foreign secretary Emily Thornberry are also in the race.
Bercow’s chances of being elevated to the House of Lords were delivered another blow today after a former Black Rod said he was submitting a complaint detailing allegations of bullying.
David Leakey said he would submit a dossier to the parliamentary commissioner for standards outlining the “intolerable rudeness and explosive behaviour” of the ex-Commons speaker.
It follows a complaint by Lord Lisvane, who was clerk of the House, which led Bercow to “categorically” deny having ever bullied anyone.
Writing in the Sunday Times, lieutenant general Leakey said he was “reluctantly” filing his own formal complaint “detailing allegations of bullying, intimidation and unacceptable behaviour” by the former Speaker.