Peter Mandelson has undermined Starmer's plans for reform of the House of Lords, warning the proposals are "not even half baked".
He said the proposals to reform the Lords were adopted by Starmer's party without a "substantive discussion" over their benefits.
Mandelson also warned that the plan - which would see an unelected Lords swapped for an elected second chamber representing the UK's nations and regions - could risk undermining proper scrutiny of legislation.
Starmer has endorsed ex-Labour PM Gordon Brown's plan for reform of the Lords.
Peter Mandelson has undermined Starmer's plans for reform of the House of Lords, warning the proposals are "not even half baked"
PA
In an interview with Lord McFall of Alcluith, the Lord Speaker, Mandelson said: "I think that we’ve got to have a far deeper conversation and analysis about this than has taken place to date.
"We haven’t had a substantive discussion about it in our own party, let alone a debate in the country.
"And yet we’re told six months away from a general election, all this is going to happen, abracadabra, in the first term of a Labour government.
"I mean there are real issues of principle."
Speaking to the Lord Speaker's Corner podcast, he added: “What I see is a sort of multi-layered cake with an assortment of very diverse ingredients in it with a thin layer of icing at the top, which is called a new second chamber of the regions and nations, which has barely been put in the oven yet, let alone fully baked.”
He told the Times that he is strongly in favour of Lords reform, but suggested Brown's proposals are more radical than he'd like.
He warned: "These proposals are too controversial to be slipped through and not debated.
"They need bipartisan support and cannot be pushed through by a majority vote of the Labour party."
Last month, it emerged that Labour plans to delay proposals to scrap the House of Lords.
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The party had previously promised to enact the plans in its first five years in Government.
It has since said it will only implement a limited number of changes, such as cutting the number of hereditary peers and reducing the total size of the chamber.
