Esther McVey's appointment as Work and Pensions Secretary is a "cause for alarm" for disabled people, Jeremy Corbyn believes, but the Labour leader condemned online abuse aimed at the new Tory Cabinet minister.
Ms McVey became a hate figure for some on the left while a minister at the Department for Work and Pensions (DWP) under David Cameron.
Shadow chancellor John McDonnell described Ms McVey as "a stain of inhumanity" during an outburst in the Commons in March 2015.
At a separate event, he also recounted a comment he had heard in which someone said the former disabilities minister should be "lynched" for her part in implementing welfare cuts.
Her return to the DWP - this time as its senior minister - led to a series of critical messages on social media.
Twitter user Red Forever, who claims to be a Labour member, branded Ms McVey a "murderess" and said she was "back to finish off her job of killing the sick and disabled".
Other social media accounts branded her a "bitch" and claimed she had "blood on her hands".
A spokesman for Mr Corbyn said: "Jeremy has made clear time and again he is opposed to all forms of online harassment or abuse. If there are such cases then that obviously should not take place."
But he added: "Esther McVey is extremely unpopular because of her record in the last government as a DWP minister and the treatment of disabled people under that government.
"It is not encouraging at all, it is alarming that she has now taken on this role with all the baggage that goes with that."
Asked if Mr Corbyn viewed her as a "stain on humanity", the spokesman said: "Jeremy is also highly critical of her record and her appointment is a cause for alarm and will be for disabled people in particular, but people across the country."
Shadow foreign secretary Emily Thornberry told BBC Radio 5 Live: "I think that those who remember what it was that she said around the time that she was cutting benefits to disabled people will be horrified to hear that she is now the Secretary of State for Work and Pensions."
It was "wrong" for her to receive death threats on social media but "what she needs to do is she needs to ensure that she educates herself properly about what the effects of cuts to benefits have on real people on a day-to-day basis".
Pressed on Mr McDonnell's 2015 comments, Ms Thornberry said: "I would never talk about anybody else like that."