By Dominic Casciani
Suspects from genocides such as Rwanda are thought to be in the UK
A parliamentary committee says an "impunity gap" is allowing some war criminals to avoid prosecution.
The Joint Committee on Human Rights said international law on atrocities is being applied inconsistently.
It also says victims of torture abroad should be allowed to use the British courts to pursue states responsible for their injuries.
Ministers have pledged reforms - but the committee said it had not seen the government's proposals.
In 2005 an Afghan warlord was successfully prosecuted for torture after he was discovered living in south London.
But while detectives have since looked at up to 30 other serious cases, including crimes against humanity, only one led to a prosecution.
One case earlier this year led to more pressure on the government after the High Court said four men could not be extradited to Rwanda because they would not be fairly tried.
A legal loophole meant the accused could not be prosecuted in the UK because the alleged offences happened before 2001.
In its report, the human rights committee says that international conventions meant the UK's courts should help to bring war criminals and others to justice, but the government had not legislated to cover all situations.
It said that some suspects were not being tried because of the time limit on when offences had occurred.
Others could not be charged because they were not legally permanently resident in the UK. But in some cases, suspects were in fact living in the UK for months or longer as business people, students, or simply waiting on an asylum claim.
Andrew Dismore, the committee's chairman, said: "The UK must not be a safe haven for evil.
"The UK should close these loopholes in the law. We also need to re-establish the specialist war crimes unit.
"Victims of torture must be able to pursue compensation, no matter who committed the crime against them. We should lead the world in bringing international criminals to justice."
The Aegis Trust, an organisation which campaigns against genocide, recently identified 18 individuals from around the world who it said should be put on trial.
It has called for suspects to be liable for arrest whenever they were in the UK, rather than just when they were legally resident.
It says that since 2004, immigration officials have recommended action on more than 420 suspects.
Justice Secretary Jack Straw has pledged to change the law this autumn, after criticism over the legal limbo of the Rwandan-four case.
A spokesman for the Ministry of Justice said the reforms would allow prosecutions for crimes as early as 1991. The ministry would also look at "providing more certainty" over prosecutions where residency in the UK is a legal issue.
"Our strong preference is for those alleged to have committed such terrible crimes to be brought to justice in the place the crimes took place so that justice can be clearly visible to the community that has suffered," said the spokesman.
"Where this is not possible, we must be able to use our own law against those who live here."