Sir Anthony Seldon told HuffPost UK the party was more divided and unsure of its future than since the end of the First World War, when it grudgingly spent another four years in coalition before ejecting its leader.
Even the 1956 Suez Crisis - the traditional benchmark of British political disasters - was not as bad for the Tories because the economy was strong and the party had a large majority and an obvious successor to a fatally weakened prime minister, Seldon added.
Seldon called May’s first 12 months “a year of reversals” and said it was “pretty difficult to find any prime minister in the last 100 years in truth who has had such a difficult first year”.
He added Gordon Brown was the only who “comes close”.
The reversal of May’s fortunes has been remarkable. She entered Downing Street on July 13 last year, taking over a party that enjoyed a huge poll lead over Labour and was united behind her.
She also sought to set out a bold vision, describing a “country that works for everyone” from the steps of Number 10 and sounding firmly in control of events ahead when she pledged “Brexit means Brexit.”
Now, she clings on, despite having decided to call an election that cost the Tories their majority and fuelled its divisions, just as Brexit negotiations begin.
Though she survives, her ministers are reportedly preparing campaigns to replace her when the time comes and Westminster’s keen observers are feverishly speculating who will replace her.
Labour’s poll lead and the absence of an obvious replacement are the main reasons she remains prime minister.
Even Anthony Eden, who was brought down by Suez and went down in history as one of the 20th century’s worst prime ministers, had a better first year than May, Seldon said.
“The Tory Party is now more in trouble than at any point for 100 years since the immediate aftermath of the First World War,” he said.
“The Tory Party is now more divided than it was over Suez, more divided than at any point [since] 1918.”
When asked whether the position of the Tories was May’s fault, he said May had inherited a difficult situation and not calling an election could also have damaged her.
“I think it’s down to the fact the country was anyway in a difficult position because Brexit was going to go wrong,” he said.
“I think she was in a ‘no-win’ situation over the General Election. If it hadn’t been called, she would be having a terribly difficult summer, people would be saying she’d blown it.”
Seldon, who has written biographies of David Cameron, Gordon Brown, Tony Blair, John Major and Margaret Thatcher, acknowledged May’s leadership style was being blamed for the election result but he said it could make her well suited to the next two years of Brexit negotiations.
Without an obvious candidate to succeed May, Seldon said the prime minister should “make it quietly known” she would leave in 2019, when the Brexit negotiations are due to finish.
Seldon has previously said May needs to yield more to her Cabinet and parliament if she is to survive.
He told HuffPost UK that, if she did this, it could play more to her strengths as a “firefighting PM” rather than one that sets the agenda with her own vision.
“She’s much more of a back foot player than a front foot player,” Seldon said.
“The questions is, can she adapt? Will the qualities that made her a very successful and long-standing Home Secretary see her through?
“All successful prime ministers have had very difficult crises and it depends how they react to them... This is going to be the most testing time of her premiership.”