Stephen Kinnock has said plans to limit child tax credits to the first two children in a family are reminiscent of "eugenics".
Labour's acting leader Harriet Harman has triggered a internal party row after she indicated she would support George Osborne's plans to cut tax credits for larger families. Three of the four candidates to succeed Ed Miliband have criticised her position.
Appearing on the BBC's Daily Politics programme today, Kinnock, the Labour MP for Aberavon, said he hoped Harman would change her mind.
Kinnock said as well as having "moral and ethical" problems with the policy it was not practical. He asked how parents would prove they deserved to be exempt from the new stricter rules, for example if a mother had become pregnant as a result of rape.
"Are we going to be asking women to go to the DWP and prove how their pregnancy came about?" he asked. "It's simply not pragmatic and it is awfully reminiscent of some kind of eugenics policy."
Eugenics, the social policy of encouraging higher birth rates among groups a society values and lower birth rates among groups society does not, is most closely associated with the Nazis.
Harman due to face Labour MPs this evening at what could be a difficult meeting of the Parliamentary Labour Party. And earlier this morning she appeared to try and walk back her comments.
A Labour source told the BBC that Harman had been setting out an "an attitude" not a final policy and would be happy for her support for cuts to tax credits to be overruled.
Labour leadership candidate Liz Kendall has offered support to Harman. But Yvette Cooper, Andy Burnham and Jeremy Corbyn have all strongly criticised their acting leader.