Some 800,000 people were killed in Rwanda's genocide
War crimes suspects from Rwanda and the Balkans living in Britain could be tried in the UK, under plans outlined by Justice Secretary Jack Straw.
Under the proposals, anyone suspected of war crimes since 1991 - 10 years earlier than the current law allows - could be prosecuted in a UK court.
Mr Straw said it would close a legal loophole and could bring forward several prosecutions.
Campaigners had warned the UK could be a "safe haven" for war criminals.
"I have received a lot of representations
that we ought to extend and make retrospective these provisions back at least another decade, for example to capture those involved in the Balkans atrocities or in the Rwanda atrocities," Mr Straw told the BBC.
"What we are announcing today is that we will indeed extend the scope of these offences back until 1991."
Mr Straw said amendments would be made to the Coroners and Justice Bill in the House of Lords in the autumn.
But he warned making the changes would be "quite complex".
Under the UK's International Criminal Court Act 2001, UK nationals or residents suspected of war crimes and acts of genocide committed anywhere in the world since 2001 can be prosecuted in Britain.
By extending the cut-off point back to 1 January 1991, the new law would apply to anyone suspected of atrocities committed in the Balkans conflict or the Rwanda massacres.
In a written statement to MPs, Mr Straw said prosecutions were best dealt with in the country of origin, or by international courts or tribunals.
"However, there may be circumstances where these options are not available. We have therefore decided that we should strengthen domestic law in this area," the statement said.
In April 2009 the High Court refused to extradite four men accused of taking part in the 1994 genocide in Rwanda, ruling there was a risk they would suffer "a flagrant denial of justice" if returned to the country.
Mass graves from the conflict in Bosnia are still being found
Afterwards, Labour MP Mary Creagh, vice chairwoman of an All-Party Parliamentary Group for Genocide Prevention, said: "Thanks to a loophole in UK law, they cannot now be tried for those crimes of which they were accused in England."
She asked Mr Straw to close the "impunity gap" and "ensure that the UK does not become a safe haven for international criminals".
Under the proposed amended law, the four could be tried in the UK.
The Aegis Trust, an anti-genocide group, welcomed Mr Straw's statement but called on the government to replace the residency requirement for prosecution of war crimes with a simple "presence test".
The trust says some suspects have been present in the UK since the 1990s but not resident because of legal reasons.
The Aegis Trust believes there are as many as 18 suspected war criminals living in Britain from countries including Afghanistan, Sudan, Sierra Leone and Sri Lanka.
In a report in June, it named Felicien Kabuga, accused of financing the Rwandan genocide, and Liberian Chucky Taylor, who was convicted of torture by the US, as two people who came to the UK but were not brought before the courts.
Mr Straw's statement on Tuesday said that under the reforms, the categories of people covered by the legislation would remain UK nationals and residents.
But he added: "However we are exploring the possibility of providing more certainty as to who may [or may not] be considered a UK resident."