According to surviving camp records, the man, now 71 and living in Florida, personally killed prisoners at the Mauthausen concentration camp in Austria.
Alexander Schweidler, a Slovak, came to Britain as a refugee in 1948, became a British subject, then emigrated to New York in 1965.
The only organisation in Britain that can consider his case is the War Crimes Unit of Scotland Yard, set up in 1989 to bring people who were not United Kingdom citizens at the time of their alleged offences under British law. 'If he has a British passport, he is allowed into the country,' a Home Office spokesman said.
His case will highlight the difference between the law on war crimes in the United States and Britain. The US deports people who concealed their past on immigration forms. In Britain, investigators have to gather enough evidence to convince a jury that the suspect committed murder or manslaughter 50 years ago.
Scotland Yard has investigated dozens of suspects living in Britain, but never brought one to court. The Office of the Special Investigator in Washington has stripped 45 men of citizenship since 1979 and deported 37 of them. An OSI official told the Independent he expected a deportation hearing - which Mr Schweidler will contest - to take place in a month's time.
Mr Schweidler now lives in Inverness, a small town in central Florida. According to the US Department of Justice, he was born in Bratislava and joined the SS in 1938. From May 1942 to 1945, he was a guard at Mauthausen, near Linz, in Austria.
In an interview with the OSI in March this year, he admitted that his duties had included shooting escaping prisoners. According to the OSI, records show that on 28 April 1942 he shot and killed two prisoners and the next day signed a statement detailing the killings.
At Mauthausen, prisoners were forced to dig tunnels for V2 rocket assembly and munitions factories. Inmates included Jews, gypsies, homosexuals, vagrants, Polish intellectuals and political prisoners, as well as American, British and Soviet prisoners of war. Tens of thousands died from shooting, gassing, hanging, electrocution, exhaustion, torture, disease, lethal injection and other forms of killing, according to the charges against Mr Schweidler.
Mr Schweidler had concealed his SS service when he applied in Liverpool for his visa to enter the US in 1965.
When he went to the US, at the age of 43, he started work as a janitor. He became a labourer, then learned about computers at night school and became a computer analyst in Connecticut. He and his wife, Anna, prospered and retired to Florida in 1984. He insists that he told both British and American immigration services about his war service.