Nick Donovan: We can hunt down the visitors with blood on their hands who are knocking on Britain's door

Those being refused citizenship are part of a wider pattern of suspected war criminals being found in the UK: since 2005, immigration authorities have taken action against more than 513 suspects and

referred 51 cases to the police.

Who are these suspects?

There are four suspected Rwandan gnocidaires still resident in the UK: including mayors accused of organising the genocide in their towns. Particularly worrying was the visit of the alleged financier of the Rwandan genocide who is accused of purchasing hundreds of thousands of machetes in 1993, and part-owning the hate radio station which directed the killers to their victims.

Suspects from Afghanistan include several warlords and a Colonel, Lieutenant Colonel and a Captain from KHAD (the communist-era security service). Of one, the immigration judge said: "I am entirely satisfied that the appellant was responsible for torturing people as part of his activities for KHAD. I consider that those activities comprise a crime against humanity and/or a war crime…"

Of a perpetrator from Saddam Hussein's regime, the immigration authorities concluded: "When cross-examined… the appellant was asked how often he had been involved in torture and he said it was on about seven or eight times. They used to beat people with cables when they did not give them the information they wanted. On average the torture would last about half an hour until they admitted the crime. Some did not admit it and they kept pressurising them for months until they gave in and gave the correct address."

Others torturers include a member of the Criminal Investigations Office in Zimbabwe under Robert Mugabe, and a Congolese police chief who even appeared in the British media to confess his crime. Another suspect who appeared on television to describe his attacks on civilians was a member of the Sudanese Janjaweed.

Other immigration judgments describe serious allegations against several Tamil Tigers from Sri Lanka, a member of the Mosquito group from Sierra Leone, officers and soldiers in Charles Taylor's army in Liberia, a Somali warlord, a member of the Serb militia "Arkan's Tigers", a rebel from Angola allegedly implicated in hostage taking, and several suspects from the Balkans.

This problem isn't unique to the UK. Instead, this reflects the sheer global scale of the problem. It is estimated that there were 200,000 perpetrators of the Rwandan genocide; 20,000 involved in the killings and ethnic cleansing in Darfur, and hundreds involved in the torture committed in Zimbabwe. It is unsurprising that a percentage of these perpetrators arrive on our shores.

So what can be done?

First, we need to re-establish a specialist war crimes unit in the British police. Following a successful campaign by local Wakefield MP, Mary Creagh, the last Labour government changed the law to allow suspected gnocidaires from the 1990s to be prosecuted. This was an important move in ending impunity.

Yet war criminals are not brought to justice by political will alone. Ensuring accountability for massacres committed long ago and far away is an arduous process.

Building up an understanding of command responsibility within communist-era secret police, Rwandan extremist militias and Mugabe's veteran's organisations requires an investment of time and resources. Even simple cases require complicated judgments about the safety of witnesses and whether a prosecution would be in the wider public interest. These complexities are part of the reason only one suspect, the Afghan warlord Commander Zardad, has been ever been prosecuted here.

However, the main reason that only one case has been prosecuted is that the police haven't been given enough resources to undertake these complex investigations. By contrast, the French government has just announced the establishment of a war crimes unit to prosecute Rwandan perpetrators. Other countries with dedicated units include Canada, the US, Sweden, Denmark, Norway and the Netherlands. The Thatcher government set up a specialist police team to investigate Nazi-era murders. This was eventually shut down in the late 1990s.

The new coalition Government could usefully re-establish a new war crimes unit for modern day war criminals.

Finally, the UK could consider the use of prosecution for immigration fraud for such suspects. If you've ever flown to the United States you may have wondered why you have to tick a box saying that you're not a terrorist, not a Nazi, and haven't committed genocide.

The reason is that the US authorities vigorously prosecute suspected war criminals and often use laws on immigration fraud to do so. If a suspect lies on their immigration application form about being, say, a concentration camp guard then they can be prosecuted for immigration fraud.

It might be a particularly fruitful approach to those suspected of committing crimes in the 1980s before the UK law change took effect. For crimes committed in the 1990s this approach is imperfect as it would be preferable to prosecute someone for the substantive crime. However, the victims at least see that their perpetrator is in court. After all, Al Capone was prosecuted for tax evasion not racketeering or murder.

In the UK, the citizenship application form now asks "In times of peace or war have you ever been involved in, or suspected of involvement in, war crimes, crimes against humanity or genocide?"

It later warns of the potential for prosecution. The Yorkshire Post has discovered 104 people who were refused citizenship because of suspicions about war crimes. The next questions for the Government to answer are: "Are there other suspects who lied on their application forms in the past? Could they be prosecuted?"

Nick Donovan is head of campaigns at the Aegis Trust, a campaign group which works to prevent crimes against humanity worldwide through research, policy development, education and raising awareness.