The widow of murdered teacher Philip Lawrence has said she was "utterly devastated" by the decision not to deport her husband's killer.
Frances Lawrence said she had been told Learco Chindamo would be deported to Italy, where his father was from.
The government said it would challenge "robustly" the decision to allow Chindamo, who stabbed Mr Lawrence in 1995 when he was 15, to stay in the UK.
The 26-year-old is serving a life sentence for the 1995 killing.
His lawyers argued that deporting him to Italy, where he was born, would breach his human rights.
The ruling allowing him to stay in the UK was made by the Asylum and Immigration Tribunal.
Chindamo, who was jailed for life in 1996 with a minimum 12-year term, could be released early next year if the Parole Board decides it is safe to do so.
Justice Secretary Jack Straw confirmed the government would be taking the case to appeal and spoke to Mrs Lawrence on Tuesday to arrange a face-to-face meeting in the next few days.
Mrs Lawrence told the BBC's Radio 4 Today programme: "I didn't anticipate that I would feel so utterly devastated as I felt.
"I think that within the confines of the law Mr Chindamo has the same rights as, for instance, my son whose father he killed.
"But what I don't understand and what makes me so depressed is how the Human Rights Act, which was set up... in order to be an exemplar to show how human beings should live together fairly and equally and kindly, and now it's allowed someone who destroyed a life to pick and choose how he wants to live his."
The human rights lawyer David Enright said on under the current rules, it was very difficult for people to be deported from one EU country to another.
Chindamo came to the UK when he was six. His father is Italian, his mother is from the Philippines and he has an Italian passport.
Mr Enright also dismissed the government's protests as "posturing", saying it had accepted the 2006 European Commission regulation into British law.
"(This) said that any EU national who had lived in the UK, even in prison, for more than 10 years, could only be removed from Britain on imperative grounds of national security - which clearly would not include Mr Chindamo or anyone like him," he added.
Mrs Lawrence said she was upset by comments made by Chindamo's defence.
"I was incensed by Mr Chindamo's lawyer. I don't want his sympathy, which I find hypocritical, or his condescension.
"This lawyer doesn't know me and yet he's implying that I'm a small-minded person, that I care only about my own feelings, and not about the wider picture."
She also spoke about the issue of forgiving Chindamo.
"This is really difficult but I think I've probably always forgiven Chindamo but it's the dealing with it that's so difficult."
Conservative leader David Cameron called for the Human Rights Act to be scrapped.
Speaking on BBC Radio WM, he said: "He [Chindamo] is someone who has been found guilty of murder and should be deported back to his country... what about the rights of Mrs Lawrence or the victim?
"The fact that the Human Rights Act means he cannot be deported flies in the face of common sense."
Chindamo was told the news at the weekend and was said to be "pleased" because his "family and life were in the UK".