Next week marks the centenary of the first Remembrance Sunday. But over the decades, hundreds of thousands of war dead who served Britain in the first world war have been written out of the story. Both the first and last shots in that war were fired not on the western front but in Africa, where British and German empires fought for colonial territories.

In addition to soldiers, Britain recruited an army of porters to carry ammunition and supplies to the front line. Conditions were appalling and many faced what amounted to a death march. It was an operation of such a scale that, to this day, former mustering points in Kenya and Tanzania are known as Kariakoo after the Carrier Corps.

Antonny Wachira Kimani is the caretaker for the Commonwealth War Graves Commission (CWGC) cemetery in Voi in southern Kenya, maintaining rows of graves near hills that lead south towards Tanzania. Paying my respects to the fallen, I read names of British captains and corporals who died far from home. I ask Kimani where the bodies of the Africans are buried. He points beyond the perimeter fence of the immaculately kept grounds into the bush, where homeless locals sometimes sleep in the undergrowth and dogs pee among rubbish.

Official records put the death toll of the Africans who served Britain in the East Africa campaign at around 100,000, but many historians estimate it at up to three times that. Kimani shows me where some of them lie. Outside the fence. No names, no graves, no dignity.

But this is not the result of Kimani neglecting his duties but of a political decision taken thousands of miles away at the highest levels of the British establishment. I had to accept that one of the British institutions I always admired, the CWGC, was hiding a scandal.

A letter uncovered in the commission’s archives, from the early 1920s, shows the rationale of those who decreed this morbid form of apartheid. It is written by the colonial secretary to the governors of the British protectorates, explaining policy that applied to the burial of “natives” who had served Britain. Unlike memorials to those who died in Europe or were white, “the commission would not erect individual headstones but a central memorial in some suitable locality to be selected by the [British colonial] government concerned”. The letter writer knew the policy well. He was the former secretary of state for war and chairman of the Imperial War Graves Commission. He concludes: “I have the honour to be, sir, your most obedient humble servant, Winston S Churchill”.

Facebook Twitter Pinterest Mwamkono Mwavaka and his wife in their garden in Kenya. His grandfather, who died in the first world war, has no known grave. Photograph: Uplands TV/Channel 4

His letter was found among bundles of incriminating documents in the archives of the CWGC, thanks to the determination of Prof Michèle Barrett of Queen Mary University of London, an expert on the history of dehumanisation. “For a long time, the War Graves Commission claimed to have acted with complete equality, and they didn’t,” Barrett explains. She has 15 years of research to back up her claims, much of it conducted in a clandestine way in the CWGC’s own archives. A decade or more ago she sent the commission an article she had published, inviting it to collaborate with her on further funded research. The response was silence.

Perhaps the CWGC had been sold the same fictitious version of history as I had. As a teenager, I visited the battlefields of the western front, then moved on to the cemeteries where Britain commemorates the bloody conflict. The Americans repatriate the bodies of their “fallen”, but Britain realised during the first world war that to do so would be an impractical and morale-lowering undertaking. So instead, the then Imperial War Graves Commission buried the men near where they died, with individual graves, according to the principle that “every one regardless of their rank or position in civil life shall be treated with equality”. When Barrett discovered that this was mainly a western front policy and didn’t apply to Africans, she resolved to call it out.

In a CWGC document, African soldiers and carriers are referred to as “semi-savage”. Another suggests “they are hardly in such a state of civilisation as to appreciate such a memorial” and “the erection of individual memorials would represent a waste of public money”. So across East Africa there are just three memorials to all of those who died, in Nairobi, Mombasa and Dar es Salaam. I reached one by clambering over a chain fence in the middle of a busy roundabout. On it is the silky verse of Kipling, “Even if you die, your sons will remember your name”.

Barrett’s detective work led us to some wasteland. She had found references to “vast Carrier Corps cemeteries” in Dar es Salaam. We were standing in one. It was entirely unmarked; no plaque, let alone a gravestone, was anywhere in sight. Beneath our feet were as many as 10,000 Africans who had served Britain. Unremembered.

A memo of a meeting between the War Graves Commission and the British colonial governor recorded: “[He] did not care to contemplate the statistics of the native African lives lost in trying to overcome the transportation difficulties of the campaign in East Africa.” Appallingly, it adds that Carrier Corps cemeteries “should be allowed to revert to nature as soon as possible”. That’s how this happened. Even today, across the world, great care is taken to track down, identify and bury British war dead. This month there will be more new burial services by the commission for recently discovered bodies. But not in Africa.

I showed Nairobi university students photographs of African soldiers and carriers from the first world war and also the manicured cemeteries for Europeans and those few Africans deemed worthy of commemoration because they were believed to be Christian. In the classroom there was bitterness. “These were brothers, uncles, fathers,” one student said.

The CWGC took part in the documentary I’m presenting for Channel 4. I met the director general, Victoria Wallace, who acknowledged that in the past the commission did not act with the kind of equality you would want to see today.

This needs to be put right, with archives scoured for names. Also the CWGC needs to start telling the story of why so many of those who served Britain are not named and not remembered as individuals.

I think the commission should start by looking at what Prof Barrett tried to insist on more than a decade ago. I’m calling for it to establish a formal group. As well as officials from the CWGC, this should include the defence secretary and representatives from the British armed forces and from Kenya, Tanzania and other former colonies.

This is the model that institutions looking to decolonise their past have embraced – accepting that the present must deal with this legacy. In a year’s time, I don’t think the CWGC website should be telling us that it has always held up a policy of equality. Because when it comes to Africa, that simply isn’t the case.

This is a story that forces us in Britain to rethink our deeply held traditions that come to the front of our minds this week and still have the power to affect our national mood, including the capacity to soothe, even momentarily, the chaos and fractures of Brexit Britain. We can’t say that we remember all our war dead with honour and dignity and yet exclude a third of a million people who died in their own continent serving Britain. Next week, as I lay my wreath in Tottenham, I will be remembering them all.

David Lammy presents The Unremembered: Britain’s Forgotten War Heroes on Sunday 10 November at 10pm on Channel 4. He is the MP for Tottenham