Ministers will move today to close a loophole that enables war criminals to use Britain as a haven from justice.

Jack Straw, the Justice Secretary, is expected to recommend changes when the Coroners and Justice Bill comes before the Lords so that prosecutions of those accused of genocide, war crimes and crimes against humanity can proceed. The gap in the law has been highlighted by Lord Carlile of Berriew, the Government’s terrorism watchdog, who has tabled amendments to the Bill.

Lord Carlile said yesterday it was important that the law could apply to atrocities committed before the date that the legislation came into force. He also believes that — in line with the model introduced in the US — the alleged criminal should simply need to be “present” in Britain, not, as some have suggested, a resident here.

Without those two provisions, the Government could be accused of being “pusillanimous”, he said. However, he was “confident that the Government’s position is moving on this and hopeful that the Government will introduce its own amendments”.

Officials had expressed fears that the reform would incur millions of pounds in prosecution costs, with scores of war criminals ending up in British courts, Lord Carlile added. But he emphasised that under his proposals, immunity for leaders of states would still prevail, and the Director of Public Prosecutions would still have to give his consent to any prosecution.

That would require both sufficient evidence, and the “public interest” to be met. “So while some people might want every general or every minister from Israel to be prosecuted, sovereign immunity would remain as a safeguard.”

Sir Ken Macdonald, a former Director of Public Prosecutions, is backing the reform. He wrote in The Times yesterday: “We already have laws that allow us to arrest visiting torturers and hostage-takers. They have been applied sensibly and haven’t led to diplomatic meltdown. Why should we be able to prosecute visiting torturers but not war criminals and genocidaires?”

According to the Aegis Trust, the anti-genocide group, 18 suspected war criminals from countries such as Sri Lanka, Iraq, Zimbabwe, Sierra Leone, Somalia and Afghanistan, have been living with impunity in Britain.

The Ministry of Justice said: “The Government believes that tackling crimes of genocide requires international co-operation. The Government will continue to work with other countries to ensure that criminal justice systems around the world are designed to deal with cases.” The spokesman added, however, that the all-party group on genocide prevention had made “a very powerful case for the inclusion of genocide as an extraterritorial offence within British law”. This was being considered.

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