Pressure is mounting on the government to compensate and apologise to Britain’s last surviving African veterans of the second world war after three shadow secretaries of state called on their Conservative counterparts to acknowledge the systematic discrimination of colonial-era troops.

Labour’s Emily Thornberry, Nia Griffith and Dan Carden – the shadow foreign, defence and international development secretaries – demanded in a letter that Theresa May’s administration acknowledge the unfair treatment, launch an investigation into the matter, issue a formal apology and pay veterans compensation.

Their intervention came after the Guardian published a document that revealed how Britain paid white soldiers – even those serving in British colonial units and living in African colonies – up to three times more than their black counterparts. The policy meant that a soldier’s skin colour and colonial origins determined pay much as his rank and length of service.

“There will therefore be righteous anger and concern amongst the British public at these latest revelations, and also a sense of urgency – given the age and relatively small number of surviving veterans affected – that they should receive at least a thorough investigation and acknowledgement into their unfair treatment, a formal apology, and if feasible, financial compensation, before it is too late,” the senior opposition politicians wrote on Wednesday.

“I am sure you all agree that these revelations are utterly reprehensible,” added the shadow secretaries of states in the joint letter, addressed to their counterparts Jeremy Hunt, Gavin Williamson and Penny Mordaunt. “When we look back at posters celebrating the joint service of men from the Empire and Commonwealth during the second world war, each of them willing to fight and die to save civilisation from fascism, we see black, white and Asian marching in step under the banner ‘Together’.

“All these years later, it is an unutterable disgrace to discover that the reward for that brave service was so callously calibrated according to the colour of those soldiers’ skin, and I am sure you will agree we must now work together on a cross-party basis, and do everything we can as a country to repair this shameful episode.”

Thornberry told the Guardian: “These servicemen and their families are owed an acknowledgement and apology for the way they were treated, but those few veterans who are still alive today are also owed a financial debt which must now be paid. Sadly, that compensation will come too late for tens of thousands of their contemporaries, but at least in death, they too will finally have justice.”

The details of the discriminatory practices were highlighted in a documentary for al-Jazeera English’s People and Power series to be broadcast on Wednesday night. The documentary also highlights how, although enlistment was supposed to be voluntary as the British government had publicly ruled out conscription, testimony from surviving veterans and their widows undermines this official line.

One veteran, 93-year-old Gershon Fundi – whom Britain sent to Ethiopia and Somaliland as a signalman – said: “They were treating us as slaves. We were there not because we wanted to be there. But we were forced to go there. If you run, even if you go home, chiefs would arrest you and then you’re going to be brought back. But how can you complain? To whom are you going to complain?”

He added: “We have no voice, we have no voice at all.”

Their testimony is supported by the research of leading historians of the era. “Pressure was put on chiefs to provide their quota and they forced men to enlist,” says David Killingray, emeritus professor of modern history at Goldsmiths University, who described the experience of individuals “caught up by this system” as appalling.

Not only were African soldiers in Britain’s forces barred from becoming commissioned officers and from disciplining lower-ranking white soldiers, they were also subjected to corporal punishment, which the British army had officially outlawed for decades.

“They beat us,” said Eusebio Mbiuki, a 100-year-old veteran who served in Britain’s Burma campaign against the Japanese. “They beat us a lot. Our bodies became so swollen from the beatings. They would beat us and slap us until you accepted everything you were being told. And you couldn’t answer back. Who would you speak to? They were your commanders.”