Five elderly Kenyans will deliver a letter to Gordon Brown tomorrow asking him to investigate what they regard as a lost decade of torture, suppression and humiliation under Britain’s colonial rule of Kenya more than half a century ago.

The three men and two women, victims of the alleged abuse, plan to visit Downing Street to hand over the letter. The move comes after the group and their lawyers issued a claim for compensation against the Government at the Royal Courts of Justice in London today.

If successful, the case could cost Britain millions of pounds by paving the way for thousands of other surviving members and sympathisers of the Mau Mau uprising to file similar claims.

Dan Leader, a lawyer for Leigh Day & Co, which is representing the Kenyans, said that he was confident of success. “We understand this is a difficult case. We understand it raises all kinds of issues but these are real people who suffered,” he said.

A leading expert warned, though, that the veterans faced difficulties in holding Britain responsible for the alleged atrocities, which include rape and castration. David Anderson, a Professor of African politics at Oxford University, said that he feared the case was being “driven by politics and money”. He said that the Government will argue that legal responsibility was passed to the Kenyan authorities after independence in 1963.

Wambugu Nyingi, 84, is one of the three men and two women who left Kenya for the first time last Friday to travel to London to open their lawsuit at the High Court. For him and the others the action is about more than compensation — they also want an apology from the Government for the alleged human rights violations.

“I lost a great deal during my time in detention,” said Mr Nyingi, who alleges that he was hit and whipped during nine years of captivity in camps run by British officers. “I was completely destroyed when I came out and couldn’t get a job.”

Jane Muthoni Mara was a teenager at the height of the Mau Mau rebellion in the 1950s. She said that nothing could fix her mental and physical scars, but that compensation and an apology would help. “At least there would be justice,” said Mrs Mara, who claims she was held at a screening camp, where she was sexually assaulted by an African Home Guard.

The Mau Mau began as a grassroots movement among the majority Kikuyu tribe to recover farmland taken by British settlers. By 1952 it had evolved into a rebellion calling for an independent Kenya. According to official estimates 11,000 Kenyans and about 100 Europeans died during the struggle, but human rights activists believe that the casualties on the Kenyan side were much higher.

George Morara, of the Kenyan Human Rights Commission, says he has evidence that proves Britain pursued a policy of torture to suppress the uprising. “Some of these things were sanctioned at the highest levels of the British Government. There is no way these were random acts,” he told The Times.

Despite the strength and gravity of the allegations, Professor Anderson is doubtful that the lawsuit in its current form — targeting the state rather than those surviving individuals who allegedly carried out the abuse — will succeed.

“There can be no doubt that torture was used by British forces . . . but the question remains who is responsible?,” he said, noting that he believes Britain will argue that legal responsibility shifted to the Kenyan Government after the transfer of sovereignty in 1963.

“A further complication is that the teams who carried out torture under the British flag included Kenyans. Any case brought against named torturers would surely have included both British and Kenyan nationals. Such actions might more readily have brought convictions, but without compensation of the kind sought by Leigh Day. The tactics here are driven by politics and money.”

The Foreign and Commonwealth Office said that it was ready to participate in an “open debate” about the past. “We understand the strong feelings that the Mau Mau issue still creates in Kenya and elsewhere. It remains a deeply divisive issue within Kenya, which historians continue to debate,” a spokeswoman said.

The other veterans suing the Government are Susan Ciongombe, who alleges that she was sexually assaulted, and Paul Nzili and Ndiku Mutwiwa, who allege they were castrated while in detention.

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