Kenyans' lawyers say discovery of files on torture and murder of detainees by colonial officials means courts can hear case.

Three elderly veterans of the Mau Mau insurgency in Kenya have launched the second round of their attempt to sue the British government following the discovery of a secret cache of Foreign Office files that document the torture and even murder of detainees by colonial officials.

Round one was won by the trio last year when the high court ruled there was "ample evidence ... that there may have been systematic torture of detainees during the [Mau Mau] emergency", and suggested it would be "dishonourable" for the courts to accept the Foreign and Commonwealth Office's argument that the veterans should be suing the current Kenyan government.

Now the FCO's lawyers are arguing the claim for compensation should be struck out because too much time has elapsed since the 1950s insurgency, with the result that a fair trial is no longer possible.

In papers submitted to the court, they argue that they face "irredeemable difficulty" in their attempts to defend the claim, not least because they cannot bring forward witnesses. "The majority who might give material evidence are now dead."

However, lawyers for the three veterans say the discovery last year of the FCO's secret archive of thousands of colonial-era files - and the realisation that many of the most incriminating documents had been systematically destroyed - means that the courts can agree to hear the case.

The archive at Hanslope Park in Buckinghamshire contained in excess of 15,000 pages of contemporaneous documents of relevance to the case, Richard Hermer QC, for the claimants, told the court. He added: "Not only is the document base vast, it is very high quality."

The veterans' lawyers also argue that some potential witnesses who could give evidence on behalf of the government are still alive.

The lawyers have retained a number of historians who argue that the claimants could not have brought a case before now because of the manner in which the abuses were systematically denied and concealed, not only by colonial officials in Kenya but by senior civil servants and government ministers in London.

One of them, Caroline Elkins, a Pulitzer prize-winning Harvard professor, submitted a statement to the court on Monday which said the wide-scale destruction of documents was part of "an explicit effort to shape an archival record and British colonial legacy in Kenya".

The documents that survived were held for decades amid strict security at Hanslope Park, the home a government research centre that provides technical help to the security and intelligence agencies.

Once the FCO acknowledged the cache's existence and allowed the veterans' lawyers to see its contents, it became clear, according to Elkins, that both Whitehall officials and the colonial administration in Nairobi sanctioned the abuse of detainees and then suppressed the evidence, and that the British army had been responsible for the "creation, maintenance and application of systems of abuse".

Another historian, David Anderson, professor of African politics at Oxford, said the files showed that one European settler, Jack Hopcraft, painstakingly documented the abuses perpetrated against his employees and that colonial officials chose to ignore him.

The newly obtained documents show that colonial officials and police officers were aware that prisoners were being raped, beaten, whipped and subjected to summary execution. Even children were killed. Eight white officers accused of "roasting detainees alive" were granted an amnesty.

One letter from 1953 shows that the colony's attorney general, Eric Griffith-Jones, expressed concern that the treatment of detainees was "distressingly reminiscent of conditions in Nazi Germany or Communist Russia".

Despite this, he drafted legislation that sanctioned a form of beatings known as "dilution technique", and then warned the then governor, Evelyn Baring, of the need for complete secrecy, because "if we are going to sin, we must sin quietly."

Senior officials in both countries were aware that this dilution technique could result in serious injury or death. But the previously secret archive makes clear that in 1959, when 11 detainees were beaten to death at a camp at Hola in south-east Kenya, the then UK attorney general, Sir Reginald Manningham-Buller, worked to shift the blame from London to Nairobi.

Although senior Red Cross officials visited Kenya and witnessed the dilution technique being applied, no mention was made of the beatings in their final report. Its author, Henri Junod, a friend of Baring, even confessed to being "impressed by the work", and told British officials that they were "angels of mercy" compared with the French in Algeria.

While an estimated one to one-and-a-half tonnes of documents were destroyed in Kenya, those that did survive and were archived at Hanslope Park include copies of correspondence generated at the Foreign Office in London which have not been transferred to the National Archive. This is raising concerns among historians that civil servants in London - as well as colonial officials - may have destroyed key documents.