Stephen Hawking was one of the world’s most accomplished cosmologists and the author of a lucrative, if often unfinished, science bestseller.

Yet newly discovered diaries, written in a secret code by his father, show that long before A Brief History of Time became a fixture on millions of bookshelves, the young Hawking was a source of paternal anxiety.

His father, Frank Hawking, worried that his son lacked drive. Writing in January 1961, when Stephen was 19 and an undergraduate at Oxford, he confided to his journal: “We are a little worried at the way Stephen is turning out. He hangs round the house with little initiative and does not study much.”

Stephen Hawking's father Frank holding baby Stephen.Frank Hawking holding Stephen in 1942courtesy of the Trustees of the Hawking Estate

The private papers of Frank and Isobel Hawking, Stephen’s parents, were uncovered during research for the first authorised biography of the physicist, to be released by the publisher John Murray in September.

Hawking, who died in March 2018 aged 76, became a symbol of academic brilliance and human resilience. Diagnosed with motor neurone disease at 21 and initially given only two years to live, he gradually lost the ability to move and speak, eventually communicating through a voice synthesiser.

His work on black holes and gravity made him one of the most recognisable scientists, while A Brief History of Time sold more than 13 million copies.

The diaries suggest, however, that at home he was not always seen as a figure of unstoppable promise.

In January 1961, Frank, an expert in tropical diseases, voiced concerns that his son was not working hard enough. “[Isobel] says he has an inferiority complex to me (he has no need to) and he has lost faith in physics at Oxford, thinking it is inferior to arts,” he wrote. “This is a great pity if so. At his age I had a burning ambition to get on, and if only I had had half his advantages, I should have done much better.”

The diaries were discovered hidden away at the home of Stephen’s sister Mary. They were made available to the author Graham Farmelo for the book Hawking: His Life and Work.

Frank Hawking kept his diary for more than 60 years, often using a code based on Greek script. Farmelo cracked the system and translated more than 200,000 words about Stephen’s childhood, illness, marriages and career.

Stephen Hawking as a child with his sisters Mary and Philippa.Stephen Hawking with sisters Mary and PhillipaSouth West News Service

A 12-year-old Stephen Hawking in a garden in St Albans, leaning on his bicycle.Hawking in his garden in St Albans aged 12Rex Features

In his diaries, Frank wrote: “This journal was originally written in Greek script to form a simple secret code and so secure greater privacy, which is essential for a journal which may fall into the hands of enemies or easily wounded intimates.”

Professor Stephen Hawking in his faculty office at Cambridge University.Hawking at Cambridge UniversityTIMES PHOTOGRAPHER Jack hill

The material appears to offer a raw portrait. In May 1967, after the birth of Stephen’s first son, Robert, Frank recorded the difficulty of spending time with his increasingly disabled son.

“I find it a slow and ghastly experience with [Stephen],” he wrote.

“Everything is so dreadfully slow and long drawn out. His speech is so slow and difficult to understand that conversation is very difficult. I am very sorry for him and will do all I can for him. But I don’t enjoy being with him.”