The mother of a schoolboy killed by a police plastic bullet has written to the Northern Ireland secretary on the anniversary of his death requesting access to a secret file about the incident.

Helen Whitters, whose 15-year-old son Paul was shot by an RUC officer during rioting on 15 April 1981 in Derry, cannot access the official papers held in the National Archives in London until the file’s scheduled release date of 2059.

In a letter to Karen Bradley, the Northern Ireland secretary, Helen Whitters pointed out she would be dead by the time the file was opened to the public.

Her son was at the front of a group of youths when an officer opened fire at close range as he bent down to pick up a stone. A Northern Ireland police ombudsman’s report in 2007 found that Paul Whitters, who was struck on the head, did not pose a threat when he was shot at under the minimum permitted range of 20 metres for firing plastic bullets. The ombudsman said the RUC had failed afterwards to obtain statements from civilians who witnessed the shooting.

Whitters was taken to hospital but never recovered from his injuries. His family said they took the “heart-breaking decision to turn off his life support on medical advice” 10 days later.

In her letter, Whitters, who now lives in Scotland, said: “I have become aware that a file exists at the National Archives in London titled Paul Whitters: killed by a plastic baton round April 1981 CJ4/4402. It appears that the file was reviewed in 2018 and the decision was made to keep the file closed until Jan 1 2059.

“I find this deeply shocking. Why should a file on the death of my 15-year-old child, which happened almost forty years ago, remain classified for another forty years? What has the British government, the NIO [Northern Ireland Office], the RUC got to hide? What secrets must be protected about events that April evening in Great James St?

“I brought Paul into this world. When this file is opened on January 1 2059 I will not be alive. No one will still be alive who actually knew Paul as the lovely, handsome, caring, intelligent young man that he was. None of his family, his friends, his classmates. Your government does not have the right to withhold this from my family. You do not have the right to withhold this from his two brothers and sister.”

Helen Whitters said her son would be 53 if he were still alive. “He might well have been a proud and loving father and husband. Who knows what he would have achieved? I owe it to him to ensure that his file is not allowed to gather dust in a vault in London simply because it is seen as too embarrassing for the establishment.”

Her campaign for justice has been supported by the Pat Finucane Centre, which has called for all files relating to plastic bullet deaths to be released.