The two-week hearing at London's High Court opens a year after a British judge ruled that Jane Muthoni Mara, Paulo Muoka Nzili and Wambugu Wa Nyingi could sue the government over allegations of atrocities including castration and torture.

The trio's lawyers say Nzili was castrated, Nyingi severely beaten and Mara subjected to appalling sexual abuse in detention camps during the bloody Mau Mau rebellion against British colonial rule.

"We are pleased that finally our clients will be able to tell the court their story," their solicitor Martyn Day said ahead of the hearing.

''The British government has thrown everything at these claims in an effort to derail them on technicalities. We are confident that justice will be done."

A fourth claimant, Ndiku Mutwiwa Mutua, has died since the High Court ruling in July last year that the test case could go ahead.

The British government is expected to argue that the claims cannot proceed because they have been brought outside the legal time limit.

But the Kenyans' lawyers, who are bringing the claims with the support of the Kenyan government and Kenya Human Rights Commission, will argue that it is an exceptional case.

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.

Nobel Peace Prize laureate Desmond Tutu has written to Prime Minister David Cameron accusing Britain of neglecting its human rights duties over the case.

The retired archbishop said Britain's unwillingness to make amends is "strongly out of step with many other modern democracies that have been faced with historic allegations of abusive conduct".