Amnesty International and other human rights groups say the UK government is handing suspected war criminals a "free ticket" in its Police Reform and Social Responsibility Bill.
As part of its new legislation, the coalition administration has decided that warrants for the arrest of people suspected of war crimes will now have to go before the Director of Public Prosecutions for approval - effectively enabling them to leave the country before action can be taken.
The change, which critics say has been "smuggled" into the new bill, is seen as a move to appease the Israeli government over attempts by lawyers to take action over alleged war crimes in Gaza.
A year ago, the Israeli opposition leader Tzipi Livni abandoned a diplomatic visit to the UK because she feared being arrested while in Britain.
The Israeli government has been lobbying for the British to amend the law since 2005, when a warrant was issued for the arrest of former military commander Doron Almog over his actions in Gaza.
He refused to leave his plane when it landed at Heathrow airport in order to avoid being seized.
But Amnesty's UK director Kate Allen says that, far from being a minor 'tidying up' change, as it is being presented, the government's move to involve the DPP "is dangerous and unnecessary".
She explained: "Unless a way of guaranteeing a means of preventing suspects from fleeing can be built into the proposals, the UK will have undermined the fight for international justice and handed war criminals a free ticket to escape justice."
The current system allows the victims of injustice to act quickly against suspected perpetrators.
In the recent past, Amnesty has also called for action on war crimes committed by Hamas through its rocket attacks aimed at civilians in southern Israel.
The scale and impact of Israeli attacks on Gaza, which it wants taken up too, is acknowledged to be massively greater, but human rights campaigners say there must be zero tolerance all round for violence against civilians.