If so his conviction could become the most expensive in British legal history. The investigation alone cost £5.4m in the first three years.

Lord Janner, chairman of the Holocaust Education Trust: "There are hundreds of war criminals out there"

Since the 1991 War Crimes Act came into force a special police unit of the Metropolitan Police has investigated 376 cases.

In nearly a third of these cases the suspects were already dead. In many more the accused were too old or too senile to be interviewed.

Another 25 were clearly established as innocent. Some names had been referred to the unit by malicious neighbours.

Thousands of pages of evidence

The war crimes unit submitted more than 250,000 pages of evidence for consideration by the Crown Prosecution Service.

The CPS seriously considered only 10 cases and the first man to be charged was Szymon Serafinowicz in 1996.

Neither Sawoniuk or Mr Serafinowicz was on a list of 17 alleged war criminals submitted to 10 Downing Street in 1986 by the Simon Wiesenthal Centre.

Those named included the head of a murder squad, the deputy chief of Latvian police in Riga and the former finance minister of the collaborationist Latvian Government.

Strict criteria

But the CPS considered each individual against strict criteria:

• There had to be reliable and admissible eyewitness evidence.

• Witnesses had to be able to clearly and uncontroversially identify the defendants.

• There had to be proof that defendants were in positions of command and not simply "following orders".

Mr Serafinowicz, a former police chief in Nazi-occupied Belarus during World War II, was the first person to be charged under the 1991 Act.

He was accused of murdering three unknown Jews.

The case collapsed because the 86-year-old retired carpenter, from Dorking, Surrey, was suffering from Alzheimer's disease.

Took his secrets to the grave

A judge ruled that he was unfit for trial. He died seven months later.

Sawoniuk was 10 years younger and the CPS felt there was little chance of a repeat performance.

Anthony Sawoniuk: jailed for life

More than 90,000 pages of evidence were submitted in Sawoniuk's case.

In the event it was enough to convict him and he is now beginning two life sentences, one of the oldest people ever to be convicted of murder in the UK.

The CPS says the case against one other individual is "under consideration" but BBC correspondents say it is unlikely to get as far as a courtroom.

There would doubtless have been more convictions if it had not been for the time gap.

Post-war 'cover up'

No war crimes legislation was enacted after the post-war Nuremberg trials because politicians and civil servants insisted no war criminals had entered the UK.

Southampton University researcher David Cesarani, a prime mover behind the Act, said: "There was absolutely no doubt that many officials and politicians knew that there were East Europeans coming into this country and a significant number of them were war criminals."

Nazi hunters continued to pressure the UK Government and the law was eventually placed on the statute book at the insistence of the then Prime Minister Margaret Thatcher.

It had been opposed by the House of Lords, who felt it would lead to "show trials".

'It was worth it'

Commander Niall Mulvihill, head of the Metropolitan Police's war crimes unit, said he did not think the investigations had been a waste of time, although he wished there had been more prosecutions.

"We have to live with what evidence we could get. We had the best possible evidence we could get, but in some of the cases there was not enough," he said.