MI6, the secret intelligence service, is under pressure to share its files on the mysterious death of a UN secretary general who was killed in an air crash almost 60 years ago.

Dag Hammarskjöld died with 13 others in September 1961, when his plane crashed near Ndola, in what was then Northern Rhodesia. There has been speculation that the plane was brought down deliberately. A film released this year, Cold Case Hammarskjöld, a winner at the 2019 Sundance film festival, has continued to stir interest.

Now a report from Mohamed Chande Othman, a former chief justice of Tanzania who was appointed by the UN to review any new information relating to the case, suggests that Britain is dragging its heels.

The UK ran intelligence agents across Africa in the 1960s and has been asked to share any information it has on the crash. “The UK and the United States must be almost certain to hold important undisclosed information,” claims Othman, adding that it took 15 months for Britain to respond to a request for information.

He adds: “Despite the United Kingdom having been identified as very likely to hold relevant information, and in spite of my indicating specific areas where such information may be found, no new documents or other information have been received, and responses were not given on the detailed queries that I communicated.”

Last year the UN asked 14 countries – including the UK, South Africa, the United States and Russia – to each appoint an independent official to conduct a review of their intelligence, security and defence archives.

The British review was conducted within a month. In his report, Othman notes: “I do not think it fair to expect that a month was adequate to accomplish a comprehensive review of this nature and covering the extensive grounds requested.” The report urges Britain and other countries with information to engage “further and deeper” with the UN investigation.

Facebook Twitter Pinterest Dag Hammarskjold in 1953. When he died he had been trying to prevent Katanga seceding from Congo. Photograph: AP

The report says it has uncovered “concrete examples” which undermine the UK’s claim to have conducted a thorough review – including a denial that it had any information relating to one Neil Ritchie, who was an MI6 agent in the Congolese province of Katanga.

Lord Lea of Crondall, chair of the Hammarskjöld Inquiry Trust, accused the British government of thwarting disclosure. “MI6 is running out of hiding space,” Lea said. “After this report they will be called out even more strongly by the UN.” He added: “The time has come for the UN to put a direct question to the UK: will they agree a meeting with MI6, yes or no?”

At the time of his death, Hammarskjöld was attempting to stop Katanga seceding from Congo, which would have had significant geopolitical consequences. Congo was receiving Soviet aid and had the world’s richest uranium reserves. In the shadows, the KGB, the CIA and MI6 all pushed their countries’ interests.

A series of inquiries has explored various hypotheses for the crash, including an aerial or ground attack, sabotage, hijacking and human error. The sole survivor, Harold Julien, said there was an explosion before the plane fell from the sky. He died from his injuries six days later.

Cold Case Hammarskjöld claims that a Belgian pilot named Jan van Risseghem had confessed to shooting down the UN plane. The Observer revealed that van Risseghem had extensive ties to Britain, including a British mother and wife, had trained with the RAF and was decorated by Britain for his service in the second world war.

• The headline of this article was amended on 14 October 2019 to substitute the word “death” for “killing”.