Theresa May was “surly” and “not particularly pleasant” during the Tories’ ill-fated general election campaign in 2017, her former joint chief of staff has claimed.

Fiona Hill was forced to become the former prime minister’s full-time minder after she “began to crumble” amid mounting criticism, according to a new biography, which says May insisted that she did not want the presidential-style campaign to be focused on her.

Historian Sir Anthony Seldon has interviewed nearly all of the most influential figures during her premiership for his book, May at 10, and he concluded that the campaign “cruelly exposed her unusually inflexible and introverted character”.

The biography, serialised in the Times, describes the initial reaction to the Tory manifesto as positive but claims that the controversy over the eventually revised social care policy – labelled the “dementia tax” – led to the unravelling of May’s credibility.

“Her team had thought the most likely manifesto backlash would come over a fuel allowance change,” he writes. “Social care had played well in focus groups when tested. [May’s election adviser Lynton] Crosby approved of it. The sceptic was Hill, who foresaw what would happen.

“It became the focus of her most bitter disagreement with [fellow No 10 chief of staff Nick] Timothy during their time together in No 10. May backed him. ‘If I don’t have it in my manifesto, it will become an empty manifesto,’ she said.”

Former aides of May told Seldon that she sought to avoid speaking to journalists or revealing personal details. “This is not what I believe in,” she would reportedly say. “I want to be out campaigning.”

Hill, with Timothy, said she was forced to travel by May’s side. “She was surly and not particularly pleasant,” she told Seldon. “She was very quiet and seemed unhappy.”

Another said: “She was a terrible campaigner. She came across as grumpy, entitled and expecting to win, and then visibly irritated when she came under scrutiny.”

The Conservatives had been expected to win a landslide victory in June 2017, but although the party increased its share of the vote it lost 13 seats and its majority as support for Labour surged.

However, after the election result May is said to have blamed her advisers and “complained bitterly that she had done exactly what they had told her to do and this was the result”.

In the aftermath, Timothy offered to resign, at which point Hill recalls May as saying: “I think you both need to resign.”

May, whose premiership lasted slightly longer than Neville Chamberlain’s, was also criticised over what was said to be her failure to show a strong face to the public after terrorist attacks in Manchester and London.

Seldon, the vice-chancellor of the University of Buckingham who has also written political biographies of Margaret Thatcher, John Major, Tony Blair, Gordon Brown and David Cameron, writes that the campaign “cruelly exposed her unusually inflexible and introverted character”.

But he reveals that May did not appear to have the final say over some of the messaging. She is said not to have liked the “strong and stable” slogan.

“I’m the leader of the Conservative party, not a presidential candidate,” May said, according to the book. “I’m not comfortable. I don’t want it to be about me.”