The government has accused the Conservatives of letting serious offenders escape justice after peers voted yesterday to change extradition arrangements with the US.
The House of Lords voted 189 to 152 to reinsert a motion removing the US from the list of countries not required to present evidence to extradite a British citizen.
MPs rejected this amendment to the police and justice bill last week by a majority of 57 but the Tories, among others, believe the extradition treaty between the UK and the US is unfair and are determined to change it.
Shadow lord chancellor Lord Kingsland said last night: "This is an important vote to win and I'm glad my colleagues in the Lords felt it was right to vote this way.
"We cannot get away from the fact that the US has never lived up to its side of the bargain on extradition and we urge the Commons to reconsider.
"The view of the Lords is that the US must provide sufficient evidence before the UK agrees to extradite those who are wanted in their country."
However, the Home Office warned that unless the proposed extradition rules were passed, six serious cases, including child sexual offences, would never go to trial. This was because the new treaty removed the time limit in which crimes can be prosecuted.
"Yet again the Tories are showing their complete lack of concern for victims of crime. By scuppering the treaty they are standing in the way of many serious criminals having to face their day in court," said Home Office minister Joan Ryan.
"Under current provisions some criminals cannot be extradited because of US laws on time limitations for criminal charges to be brought.
"The Tories have voted for measures that prevent us from bringing criminals to book, and give victims of some of the most horrific crimes the chance to see justice done."
Under the 2003 extradition treaty, British prosecutors must provide evidence before extraditing US citizens to face trial in the UK, but their American counterparts need only offer "information" to bring British citizens to the US.
The government insists the two requirements are broadly similar, but critics argue it is in fact much easier to extradite someone from the UK to the US than vice versa.