The answer to one of the great mysteries of the Second World War will be revealed at a lecture in Bradford.

The abrupt end to the wartime broadcasts of Bradford writer JB Priestley has created intrigue for more than half a century.

Now his stepson believes he has come up with the definitive answer as to why the Sunday evening radio programme, designed to rally morale among the population, suddenly ended in 1941.

Nicolas Hawkes, of Kings Road, Ilkley, has scoured the BBC archives and other documents from the time in order to end the speculation.

He said: "I have investigated the story from the point of view of political intervention which led to his series being terminated at that time.

"His broadcasts rallied and unified the nation like none other than Churchill. He had the power of language combined with his natural Yorkshire common touch. He had a certain magic and he made a reputation for himself."

Mr Hawkes said his stepfather had never spoken about the

way in which the broadcasts ended.

But he says the evidence suggests that Priestley's scripts finally prompted elements of the political establishment to take action.

Mr Hawkes, a retired lecturer, said: "He did believe in co-operation and in building a better and more just society after

the war and that's why a few people on the right did not like him.

"There were some Conservatives who were just bothered about winning the war, but he was concerned about how to improve life after the war."

Mr Hawkes said his stepfather was left of centre not politically but in his outlook on society.

JB Priestley began his broadcasts in the aftermath of the Dunkirk evacuation and remained on the airwaves for 20 weeks.

He was a person who could reach people across the country - partly thanks to his reassuring voice - and he was

praised and admired in many quarters.

Mr Hawkes will deliver his lecture at Bradford University on Saturday, March 18. It will include extracts from Priestley's surviving recordings along with photographs. The exact reason why the broadcasts ended will also be revealed, said Mr Hawkes.

Mr Hawkes described his step-father as a strong character who could be surprisingly gentle and even shy at times.

"But when he was in a determined mood, he was very powerful," he said.

"He wrote a lot and was a brilliant playwright. His broadcast talks were spoken essays which took up little observations he made as he went round the country."

Mr Hawkes is an active member of the JB Priestley Society, which has been instrumental in arranging the lecturer.