A court in London has been told the British government accepts that colonial forces in Kenya in the 1950s and 60s tortured and abused detainees during the Mau Mau rebellion.
Three elderly Kenyan men are suing the government for damages. They want an official apology and damages to set up a Mau Mau welfare fund.
Their lawyers say it is the first official acknowledgement by Britain that torture was used.
The BBC reports the revolt against British rule in Kenya in the 1950s and 1960s was marked by atrocities, with thousands killed.
The British government argues that too much time has passed for a fair trial to be conducted.
Before starting cross-examination of witnesses at the trial, Guy Mansfield QC said he did not dispute that civilians had suffered "torture and ill-treatment at the hands of the colonial administration".
He spoke directly to each of the witnesses, saying he did "not want to dispute the fact that terrible things happened to you".
Papers in the test case were first served in 2009. In 2011, a High Court judge ruled the claimants - Paulo Muoka Nzili, Wambuga Wa Nyingi and Jane Muthoni Mara - did have an arguable case.
Their lawyers say that Mr Nzili was castrated, Mr Nyingi severely beaten and Mrs Mara subjected to sexual abuse in detention camps during the rebellion.
A fourth claimant, Ndiku Mutwiwa Mutua, has died since the High Court ruling that the test case could go ahead.
Procedural ruling awaited
In his ruling in 2011, Mr Justice McCombe emphasised he had not found there was systematic torture in the Kenyan camps nor that, if there was, the British government was liable for what had passed.
It will now be decided whether a fair trial is still possible.
The hearing will have access to an archive of 8000 secret files that were sent back to Britain after Kenya gained its independence in 1963.
Earlier in July, South African Archbishop Desmond Tutu wrote to Prime Minister David Cameron accusing Britain of neglecting its human rights duties over the case.