Green MP Golriz Ghahraman worked as part of the legal defence team for Bosnian Serb wartime leader Radovan Karadžić, who was found guilty of genocide and crimes against humanity.
Party co-leader James Shaw is standing by his MP, saying her work on international tribunals as both a defence and prosecution lawyer is all part of a robust justice system.
But her profile page on the Green Party website has now been changed to more accurately reflect the legal defence work she did at the Rwanda Tribunal and The Hague, and the prosecution work she did at the Khmer Rouge Tribunal.
In a series of tweets this week, former Labour staffer Phil Quin criticised Ghahraman's work at the Rwanda Tribunal, saying she volunteered to defend "the worst killers known to man" and calling her a genocide-denier.
"Any MP who acted as a voluntary intern to defend war criminals, and authors papers that deny the Rwandan genocide, must resign," said Quin, who lived and worked in Rwanda for several years.
In a comment piece on Newsroom, he added: "It's one thing for a UN defence lawyer to be assigned to defend ratbags. It's quite another to seek them out in a voluntary capacity."
Ghahraman worked as an unpaid intern as part of a team that defended Joseph Nzirorera, who died before he could be convicted of genocide, and in a paid position as part of a team representing pop singer Simon Bikindi, who was convicted for incitement to genocide.
Quin published a photo of a smiling Ghahraman with Bikindi on Twitter today.
Shaw said Quin's attacks were politically motivated.
"I think Phil knows as well as anybody that a functioning justice system requires both a rigorous prosecution and a rigorous defence in order to make sure that the trial actually delivers the result its intended to."
Ghahraman said everyone was entitled to a fair trial, "including those accused of very egregious crimes".
"I would essentially be letting down the human rights framework if I thought anything other than that. My work on defence and prosecution has always been equally a point of pride for me."
Her profile page on the Green Party website has now been changed, following her admission that it "could be clearer".
It previously said: "Golriz has lived and worked in Africa, The Hague and Cambodia, putting on trial world leaders for abusing their power."
Now it says: "Golriz worked for United Nations Tribunals as part of both defence (Rwanda, the former Yugoslavia) and prosecution (Cambodia) teams."
But she said she has never hid her defence work, and that it's "certainly not something I'm ashamed of".
"It's absolutely offensive to say that I deny genocide, because there's nothing that's been more important to me than to highlight genocide as an international crime."
Legal expert Andrew Geddis came to Ghahraman's defence today. Writing for the Pundit blog, he wrote: "Ghahraman played a necessary (if hard) role in an internationally established institution designed to resolve in an open and legitimate fashion individual guilt for horrible actions.
"Defending nasty individuals is just a part of what international human rights lawyers do."