The diary of Captain Stephen Alley, second in command of the British intelligence mission in Petrograd — now St Petersburg — shows he positioned four undercover agents ready to extract what he called “the valuables” — the deposed Tsar Nicholas II and the Russian imperial family — from the House of Special Purpose where they were held.

The diary also includes a sketch map drawn by Alley of the house and its surroundings.

It used to be believed that Britain had abandoned the tsar, his wife Alexandra — a granddaughter of Queen Victoria — and their children. But in recent years evidence has emerged that both King George V and the government of David Lloyd George were willing to rescue the family. No evidence has previously come to light, however, of the advanced stage that preparations had reached.

Alley’s diary was found accidentally by his descendants in a trunk of his papers and will be featured in Queen Victoria’s Grandchildren, a documentary to be shown on Channel 4 in December.

The diary shows that, after they had been sprung from custody, the tsar and his family were to be taken by train to Murmansk and then shipped to safety by the Royal Navy.

On May 24, 1918, Alley, who was employed by MI1 (c), part of what became MI6, wrote to the War Office in London naming the six Russian- speaking officers he wanted to carry out the rescue. He asked London for a grant of £1,000 a month (about £25,000 today) due to “increased requirements for intelligence purposes”.

Andrew Cook, the historian who has examined the papers for the documentary, believes Alley’s telegrams to London may have been intercepted, leading the Bolsheviks to reinforce defences around the tsar’s prison. “At the first hint of a rescue the whole family would have been shot,” he said.

Alley’s apparent reluctance to activate the plot led to his sacking and recall to Britain. He worked for MI5 in the second world war and died in 1969 at the age of 93.

He always kept his work secret, even from Beatrice, his wife. Anthony Summers, author of The File on the Tsar, said: “She told me that when she asked her husband what he did, he would say, ‘Sometimes I will go away for a night and sometimes for a year, and I won’t be able to tell you where I am, but I’m working for the king.’ She thought he was going for dirty weekends.”

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