The men deny charges of genocide and crimes against humanity
Four men have won a High Court battle to halt extradition to Rwanda where they face mass murder charges.
Senior judges ruled that there was "a real risk" they would suffer "a flagrant denial of justice".
All four are wanted in connection with the 1994 genocide in which 800,000 people were killed in 100 days.
The men deny involvement in the genocide and say they would not receive a fair trial if they were forced to return to Rwanda.
Anti-genocide campaigners condemned the court's decision, saying that it sent a message to the Rwandan people that the suspected killers of their families could "walk free with impunity".
The court ordered the four men, who have been held in custody since December 2006, be released.
They are: Dr Vincent Bajinya, from north London; Charles Munyaneza, from Bedford; Celestin Ugirashebuja from Essex; and Emmanuel Nteziryayo, of Manchester.
This decision demonstrates to the Rwandan people that the suspected killers of their families can walk free with impunity
Dr James Smith of the Aegis Trust
The men are accused of killing, or conspiring with or aiding and abetting others, to kill members of the Tutsi ethnic group "with the intent to destroy in whole, or in part, that group".
Mr Munyaneza, Mr Ugirashebuja and Mr Nteziryayo were said to be mayors of local communes who allegedly organised killings in their areas while Dr Bajinya was allegedly a militia organiser in Kigali.
At an extradition hearing in 2007, a court heard allegations that Dr Bajinya, who changed his name to Vincent Brown when he became a British citizen, had been part of the "inner circle" of then Rwandan president Juvenal Habyarimana.
After the president died in a plane crash in 1994 Dr Bajinya - a former member of the Rwandan governing MRND party - was claimed to have attended key "genocide meetings" where plans to murder hundreds of thousands of Tutsis were planned.
The court was told Dr Bajinya then went on to become a leader in the Interahamwe militia, which was to spearhead the slaughter.
Court papers alleged he ordered the militia to cut a suspected Tutsi "into pieces so that he would not recover".
Dr Bajinya is also accused of personally interrogating a Tutsi woman about where her fellow "inyenzi" - or cockroaches - were before a militia man shot her dead.
Lord Justice Laws and Lord Justice Sullivan, sitting at London's High Court, allowed the appeals by Dr Bajinya and his three co-defendants against the extradition ruling by Home Secretary Jacqui Smith because, the judges said, there was evidence that defence witnesses in Rwanda were afraid to give evidence in the men's favour.
The judges added: "We conclude that if (the four) were extradited to face trial in the High Court of Rwanda, the appellants would suffer a real risk of a flagrant denial of justice by reason of their likely inability to adduce the evidence of supporting witnesses."
The judges also ruled there was a real risk "of executive (government) interference with the judiciary" in Rwanda.
They refused the Rwandan government, represented by the Crown Prosecution Service, permission to appeal to the House of Lords against their ruling.
The judges' decision is thought to be the first time an English court has ever blocked an extradition request from a foreign government on the grounds that it would violate Article 6 of the European Convention on Human Rights, which safeguards the right to a fair trial.
Lord Justice Laws described it as "an important and anxious case". Lord Gifford QC, who had appeared for Mr Munyaneza, said the case had revealed "an emerging international consensus that there is no fair trial in Rwanda".
However there was criticism that a legal loophole had allowed four suspected mass murderers to walk free.
Dr James Smith of the Aegis Trust, which campaigns against genocide told the BBC that the UK government should change the law to allow British courts to prosecute crimes against humanity.
He said: "If the High Court is unable to allow extradition because the legal system in other countries does not meet its high standards, then the government should revise and amend legislation to allow proceedings in this country.
"As it stands this decision demonstrates to the Rwandan people that the suspected killers of their families can walk free with impunity in Europe."
An estimated 800,000 Rwandans were murdered in about three months.
Frank Brazell, a solicitor representing Vincent Brown aka Dr Bajinya, welcomed the judges' ruling, saying: "We are hugely pleased with the result.
"The central issue they have found is that there is clearly no prospect of these men having a fair trial in Rwanda.
"The primary reason is the likelihood of political interference affecting the approach taken by the judges of Rwanda to a case of this kind.
"Obviously this is linked to interference with witnesses and the inability to call defence witnesses because of government or judicial interference."
He said Mr Brown, a British national and qualified doctor who had worked for a charity training nurses, would soon be released from custody.
He has been held since 28 December 2006. Asked if Mr Brown would now seek compensation, Mr Brazell said the issue would be discussed in due course.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.