Pressure is building on the British and American governments to respond to claims that they possess secret information that would shine light on the mysterious death of a former UN secretary general in Africa more than 50 years ago.

Last month a UN report into the death of Dag Hammarskjöld in a plane crash in 1961 found that there was a “significant amount of evidence” that the Albertina DC6 in which the Swedish diplomat was flying had been brought down in a forest near the city of Ndola in northern Rhodesia, now Zambia, by another aircraft.

The report took into account previously undisclosed information provided by the US, UK, Belgian, Canadian and German governments and noted that the US and UK governments were in possession of intercepted radio traffic in the area at the time.

Dag Hammarskjold died in the 1961 plane crash. Photograph: AP

Now Lord Lea of Crondall, chairman of the trustees who established the Hammarskjöld Commission – which was instrumental in persuading the UN to reinvestigate the death – has written to the foreign secretary, Boris Johnson, and the US secretary of state, Rex Tillerson, saying that, in light of the UN’s report, the UK and US government must now state publicly whether or not they are in possession of the intercepts.

In his letter to the two senior politicians, Lea says it is “unthinkable that there are no transcripts [of the intercepted messages] in the US archives and perhaps also in the UK archives”.

He adds: “I suspect that there is so far undisclosed and highly pertinent information in the transcripts. When the US authorities replied to the Hammarskjöld Commission’s very precise request for transcripts, they said that they had ‘documents responsive to your request’ but claimed that they were secret.”

At the time of his death, Hammarskjöld was attempting to unite Congo and stop the province of Katanga seceding. Mining firms feared their concessions would be jeopardised if Katanga gained independence.

A former Belgian pilot known only as Beukels has claimed that he accidentally shot down the Albertina in an attempt to divert it from Ndola to another airport. The report by the Hammarskjöld Commission suggested that “a group representing a number of European political and business interests ... wanted the secretary general’s plane diverted from Ndola ... in order to persuade him of the case for Katanga’s continued independence”. Beukels said he shone a bright light down from his Fouga jet on to the DC6 while his radio operator called on the plane to divert. After failing to receive a reply, Beukels has said that he fired several warning shots, at least one of which clipped the Albertina’s wing, causing it to crash.

It is thought that US and possibly UK intelligence stations in Cyprus were monitoring radio traffic in the area at the time.

In his letter, Lea claims: “The time is now long overdue – more than 50 years after this tragic event – for the US and the UK to conduct a complete review of records in their custody – including in particular still-classified records – and make them available to the United Nations.”