The Parliamentary and Health Service Ombudsman (PHSO) has ordered the Department of Work and Pensions to pay Adrian Furnival £675 for the injustice he suffered.
The Ombudsman said Government bodies needed to ensure that their communication with the public was always fair, clear, and consistent.
Mr Furnival, 82, and his wife Sheila, 67, moved to Brittany in 1994.
In 2018, Mr Furnival found out, via an annual statement from that from 2020, he would no longer receive Adult Dependency Increase (ADI) payments, a supplement given to households when the main earner reached age, but their partner had not.
People in the UK were told about the change, which meant he would be over £250 a month worse off eight years earlier in 2010.
PHSO found that the Department failed to properly communicate the changes to Adrian, and the should have told him about them in April 2010.
The Ombudsman recommended that apologise and pay Adrian £675 for the injustice he suffered.
The number of those living abroad and entitled to ADI is unknown, but in May 2019, a year before ADI ended, told Parliament that 10,817 people still received ADI.
The ombudsman recommended that the provide a remedy comparable to anyone who approaches the Department in a similar situation.
Rebecca Hilsenrath, Parliamentary and Health Service Ombudsman said: "Poor communication from Government departments damages trust in public services.
" has a history of failing to communicate pension policy changes clearly and failing to learn from its mistakes. In Adrian's case, this meant that, without the right information, he lost the opportunity to prepare for his retirement. It also caused him unnecessary financial worry.
"Anyone who believes they have had a similar experience to Adrian should contact . has complied with our recommendations and will provide a comparable remedy to anyone who approaches them with a similar situation."
Mr Furnival, who was born in Bedford and served in the Army, said the news of the change came as a shock.
"If they had told us at the same time as everyone else, we could have had eight more years to plan for the shortfall. That would have given us enough time to do something; my wife or I could have tried to get a job. By 2018, we had no way of replacing that income.
"This could have been handled much better, and I knew that it could well affect other people in the same situation. When I first queried it with , they took nine months to reply, and it felt like we just went round and round. Which is why I then raised it with my MP and brought my complaint to the Ombudsman."
In December, accepted PHSO's finding of maladministration in communicating changes to the age to women born in the 1950s and apologised.
The Department said it will learn lessons and work with the Ombudsman to create an action plan to make sure future changes are communicated well.