LONDON — Renford McIntyre was just a child when he left Jamaica to join his parents in Britain.

He spent the next 50 years here, working various jobs and paying taxes. Like tens of thousands of people from families encouraged to come from British colonies in the Caribbean to help rebuild postwar Britain, he had always assumed that he was a full-fledged British citizen.

Until, that is, the government decided he wasn’t. At the age of 60, he was declared an illegal immigrant. He lost his job. He could no longer apply for benefits. He became homeless.

“I’ve lost out on everything,” said Mr. McIntyre, who is now 64.

The issue has simmered in Britain since Theresa May was home secretary and set out to create a “really hostile environment” for illegal immigrants, imposing tough new requirements in 2012 for people to prove their legal status. But that was not easy for thousands of people like Mr. McIntyre, many of whom did not have British documents and whose parents came to Britain before the Caribbean colonies gained independence.

Just this month, as leaders of former British colonies gathered in London for a Commonwealth meeting, Mrs. May, now prime minister, found herself forced to apologize for the harsh treatment many Caribbean immigrants were experiencing and pledged to grant citizenship to all those caught up in the bureaucratic tangle.