More than half a million black African soldiers who fought in the British army during the second world war were paid up to three times less than their white counterparts, a newly unearthed document has revealed, prompting calls for an investigation and the government to compensate surviving veterans.

The document, buried in Britain’s national archives, reveals how the government systematically discriminated against African soldiers, paying white personnel – even those living in African colonies and serving alongside African soldiers in British colonial units – far more than their black counterparts.

The international development secretary, Penny Mordaunt, acknowledged that inequalities existed in the past.

Labour MPs are calling for a government inquiry into the matter. On Wednesday, Wayne David, the shadow defence minister, demanded an immediate official investigation: “The defence secretary ought to make an announcement to the House of Commons and put on the record that this was wrong and that he’ll put it right – as simple and straightforward and emphatic as that … There needs to be a full-scale government inquiry and all the information needs to be brought forward.”

The document was uncovered by the makers of a documentary for Al Jazeera English’s People and Power series. It reveals that Britain paid its soldiers not only according to their rank and length of service but also the colour of their skin.

A table from an official British government document, published in 1945, outlining the pay scale for various races. Photograph: Kew Archives

“The colonial regime placed a different value on African life than it did on European life,” said Prof Timothy Parsons, one of the world’s leading authorities on Britain’s east African army, who is calling on the government to redress the injustices exacted on these soldiers by British rule. “There may be an opportunity to right some wrongs. It wouldn’t cost a great deal of money per individual.”

Once a soldier was demobilised, Britain paid him a lump sum known as a war gratuity, calibrating the exact amount to the racial hierarchy enshrined in its African empire.

“When I got out, they gave me nothing,” said Eusebio Mbiuki, a 100-year-old veteran who lives in poverty in rural Kenya, having endured brutal combat in the jungles of Burma while fighting for Britain. “They should have known how much we had helped them. They would have given something. But that was not the case. We were abandoned just like that.”

Throughout the first half of the 20th century, Britain recruited African men — forcibly in numerous cases — to protect its colonies on the continent and further afield. During the second world war, these soldiers bolstered Allied forces to defeat the Italians in the Horn of Africa, to capture Madagascar from the Vichy French and to fight Japan’s imperial forces in desperate conditions across Burma.

Despite their sacrifices and harrowing deployments, the British government denied these African soldiers equal treatment. The Al Jazeera investigation, due to be aired on Wednesday night , reveals that a white private could earn 10 shillings for each month of service, compared with just three-and-a-half shillings on offer to a black soldier of the same rank.

For each white corporal, Britain put his war gratuity at 12 shillings per month of service, and just four shillings per month for black corporals. Each payment was made by Britain’s various colonial administrations, setting the figure in consultation with the War and Colonial Offices in London.

Rewards were even more abysmal for Africans recruited as labourers, even though these supposed non-combatants regularly found themselves in the firing line. Privates, corporals and sergeants in this labour corps could respectively receive two, two-and-a-half and three shillings per month of service — around one fifth of the amount given to white soldiers.

Britain’s racial discrimination extended to Asian personnel recruited in British east Africa too. Irrespective of rank, these troops all earned seven-and-a-half shillings for each month of enlisted service: less than their white comrades, more than their African ones.

The former chief of the UK armed forces, General Lord Richards, shows a photograph to children in northern Zambia during a visit to the last battleground of WWI in Africa. Photograph: Jack Losh

A former chief of the UK armed forces, Gen Lord Richards, condemned the neglect of these soldiers following their service in the British army.

“We’re talking about people who fought for us in the most horrendous circumstances,” he told the film-makers. “Surely if Britain and the British people mean anything, it’s about generosity of spirit and I feel our political leadership sometimes doesn’t remember that.”

Richards added: “We should be ashamed that veterans who fought for our country are living in that poverty … It’s not too late. We can still make amends.”

Prince Harry attends a meet-and-greet with war veterans and war widows at Burma Barracks in Lusaka, Zambia. Photograph: Jack Losh

Mordaunt, whose department, through UK aid, has supported some Commonwealth veterans and their widows who live in extreme poverty, conceded that Britain’s colonial-era forces had faced significant discrimination.

“There were obviously tremendous inequalities in how people were treated,” said the cabinet minister, who last November announced a new aid package to help impoverished veterans and war widows from Commonwealth nations. “Clearly there were prejudices that existed many years ago, that were very prevalent at the time, that were causing people to be treated very differently – people not to be appreciated for the service that they gave.”

Preet Gill, the shadow international development minister, backed calls for the government to compensate surviving veterans: “The prime minister has to come back to the House, acknowledge that this exists, apologise for it, and set out what she’s going to do. She has a duty on behalf of the country to come out and recognise this. Any person leading our country has to do this – it doesn’t matter how many years later. It’s only been 70 years and these men are still alive. She needs to meet with them and their families and acknowledge what this actually means for them. They weren’t able to do anything in that time to raise their voice.”

Menzies Campbell, a former Liberal Democrat leader and defence spokesman, said: “A life is a life, for God’s sake. Any life is worth the same value, … If those who served our country are in poverty now then surely there is time to recognise that their sacrifice and their service was of equal value as anyone else in the British army. They deserve both recognition and recompense now. Viewed from today, the distinction was wholly unjustified.”

A Ministry of Defence spokesman said: “The UK is indebted to all those servicemen and women from Africa who volunteered to serve with Britain during the second world war. Their bravery and sacrifice significantly contributed to the freedoms that we all enjoy today.”

Forgotten Heroes of Empire, a film by Jack Losh and Alessandro Pavone, will air on Wednesday on Al Jazeera English at 10.30pm

• This article was amended on 14 February 2019. An earlier version said the Department for International Development was responsible for the welfare of Commonwealth veterans. This is not the case, though the department, through UK aid, has supported some veterans and their families.