LONDON: Britain on Thursday offered an "unreserved apology" after a new report said hundreds of thousands of African and ethnic minority soldiers who died fighting for the British Empire in World War I were not properly commemorated due to "pervasive racism".

Prime Minister Boris Johnson said he was "deeply troubled" by the findings of a Commonwealth War Graves Commission (CWGC) probe, which suggested soldiers were not treated as fairly in death as their white British counterparts.

"During the First World War, millions of people from Africa, Asia, the Caribbean and the Middle East fought for Britain in the struggle against tyranny," he said in a statement.

"Their contribution to victory was immense, not just in numerical terms but in their courage and valour, and many paid the ultimate price so that we might live in peace and freedom today.

"I am deeply troubled by the findings... that not all of our war dead were commemorated with equal care and reverence. On behalf of the government, I offer an unreserved apology."

Johnson's statement followed Defence Secretary Ben Wallace's verbal apology in parliament as Britain reckons with the legacy of its colonial past and links to slavery, against a backdrop of anti-racism and Black Lives Matter protests.

Last month, there was a backlash at a government-commissioned report which concluded prejudice persists but the country is not "institutionally racist", enraging equality campaigners.

The CWGC said the failure to honour the troops was "influenced by a scarcity of information, errors inherited from other organisations and the opinions of colonial administrators".

"Underpinning all these decisions, however, were the entrenched prejudices, preconceptions and pervasive racism of contemporary imperial attitudes."