Shami Chakrabarti, the shadow attorney general and a close ally of Jeremy Corbyn, condemned the comments by newly-appointed Defence Secretary.
Gavin Williamson had suggested UK fighters should not be allowed to come home, telling the Daily Mail: "A dead terrorist can't cause any harm to Britain. I do not believe that any terrorist, whether they come from this country or any other, should ever be allowed back into this country.
"We should do everything we can to destroy and eliminate that threat."
But Baroness Chakrabarti, a former civil liberties campaigner, said the comments were "very disappointing" and that jihadis should be treated like other criminals and brought to justice.
She told Sky's Niall Paterson: "If you're on the battlefield then you have to fight whoever is opposing you on the battlefield.
"If he's suggesting targeted assassinations of criminals who should instead be brought to justice, that is appalling and we do not want to hear a Defence Secretary abrogate the rule of law.
"Because the rule of law is what brave men and women who join our armed forces and put their lives at risk, that's what they do it for - for the rule of law at home and abroad."
Mr Williamson's comments sparked a row when former director of public prosecutions Lord MacDonald, who sits as a Liberal Democrat peer, branded them "a juvenile response" that "belong in a Netflix series".
But the Defence Secretary appeared to have Downing Street's backing.
The Prime Minister's official spokesman said this week that the Government's policy was that "if you travel to Iraq and Syria and if you are fighting with our enemies there then you make yourself a legitimate target".
More than 800 UK citizens are thought to have gone to fight for IS in Iraq and Syria. Around half have returned, some 130 have been killed and an estimated 270 are thought to be still in the field.
Two years ago, Jeremy Corbyn responded to the death of IS executioner Jihadi John - real name Mohammed Emwazi - in a targeted air strike by saying it would have been better to hold him to account for his "callous and brutal crimes" in court.
Mr Williamson has also been hitting the headlines weeks into his new role amid reports of clashes with chancellor Philip Hammond.
Mr Hammond's allies have compared Mr Williamson to Private Pike, the naive character in Dad's Army.
The Defence Secretary's allies appear to have retaliated, as a story emerged that the Treasury had failed to pay for the Chancellor's use of an official RAF plane.