Image caption Capt Campbell travelled by boat and train to visit his sick mother in Gravesend, Kent

A British officer captured during World War I was granted leave to visit his dying mother on one condition - that he return, a historian has discovered.

And Capt Robert Campbell kept his promise to Kaiser Wilhelm II and returned from Kent to Germany, where he stayed until the war ended in 1918.

Historian Richard van Emden told the BBC that Capt Campbell would have felt a duty to honour his word.

It also emerged that Capt Campbell tried to escape as soon as he returned.

Mr van Emden discovered the story when researching Foreign Office documents at the National Archives for his book, Meeting the Enemy: The Human Face of the Great War.

Twenty-nine-year-old Capt Campbell, of the 1st Battalion East Surrey Regiment, had been captured in northern France on 24 August 1914 and then sent to a prisoner-of-war camp in Magdeburg, north-east Germany.

While in the camp he received news that his mother Louise was dying of cancer.

Attempted escape

Capt Campbell wrote a letter to the German emperor begging to be allowed to go and see his mother, which the Kaiser allowed - as long as Capt Campbell gave his word that he would return.

Mr van Emden said that Capt Campbell almost certainly travelled through the Netherlands and then by boat and train to Gravesend in Kent, where he spent a week with his mother before returning to Germany the same way.

His mother died in February 1917.

Mr van Emden told the BBC that Capt Campbell would have felt a duty to honour his word and "he would have thought 'if I don't go back no other officer will ever be released on this basis'".

Mr van Emden said it was "surprising" that Capt Campbell was not blocked from returning to Germany from Britain.

No other British prisoners of war were afforded compassionate leave, though, after Britain blocked a similar request from German prisoner Peter Gastreich, who was being held at an internment camp on the Isle of Man.

In another twist to the story, Mr van Emden said that as soon as Capt Campbell returned to the camp he then set about trying to escape.

He and a group of other prisoners spent nine months digging their way out of the camp before being captured on the Dutch border and sent back.

Mr van Emden said that as well as feeling honour bound to keep his word to return to the camp, as an officer, Capt Campbell was also honour bound to try to escape.

The Daily Mail reported that after the war Capt Campbell returned to Britain and served in the military until 1925.

He then rejoined when World War II broke out in 1939, serving as the chief observer of the Royal Observer Corps on the Isle of Wight.

He died in the Isle of Wight in July 1966, aged 81.