Former US Vice-President Kamala Harris has delivered her sharpest criticism yet of her former boss, calling Joe Biden's decision to seek a second term "recklessness" in an excerpt from her forthcoming memoir.

"'It's Joe and Jill's decision.' We all said that, like a mantra, as if we'd all been hypnotized," Harris writes in her book. "Was it grace, or was it recklessness? In retrospect, I think it was recklessness."

In an extract for her book 107 Days, published by The Atlantic on Wednesday, Harris also describes moments where she felt sidelined or denied credit for her work by Biden's team.

The BBC has contacted Biden's office for comment.

Harris wrote that as vice-president she was in the "worst position" to tell Biden not to run for president again.

"I knew it would come off to him as incredibly self-serving if I advised him not to run," she wrote. "He would see it as naked ambition, perhaps as poisonous disloyalty, even if my only message was: Don't let the other guy win."

The Atlantic published a 3,000-word excerpt from Harris's book – the title is a reference to the length of her failed presidential campaign. The book will be published in full later this month.

Biden withdrew from the 2024 race following a dismal debate performance against then Republican candidate Donald Trump.

The debate performance fuelled questions about Biden's age and mental fitness to lead the country. Harris eventually lost the election to Trump.

Harris wrote that 81-year-old Biden's choice to run for re-election "should have been more than a personal decision".

"The stakes were simply too high. This wasn't a choice that should have been left to an individual's ego, an individual's ambition," she wrote.

She denied that there was a "big conspiracy" to hide Biden's frailty and described the former president as "a smart guy with long experience and deep conviction, able to discharge the duties of president".

"But at 81, Joe got tired. That's when his age showed in physical and verbal stumbles," she wrote.

Harris also alleges the White House failed to adequately respond to her critics.

The former vice-president recalled securing billions of dollars in investment commitments from private companies for Latin American countries to help tackle the root cause of migration.

Despite this, Harris wrote, Republicans "mischaracterized my role as 'border czar'" - a description that dogged her during her presidential campaign as the number of illegal border crossings spiked.

"No one in the White House [communications] team helped me to effectively push back and explain what I had really been tasked to do, nor to highlight any of the progress I had achieved," she wrote.

Harris also described a trip she made to Texas in July 2024, in the wake of a devastating hurricane, and listening to a televised address by Biden while in a hotel room in Houston.

"It was a good speech, drawing on the history of the presidency to locate his own place within it," she wrote. "But as my staff later pointed out, it was almost nine minutes into the 11-minute address before he mentioned me."

Biden and Harris both ran for the Democratic nomination in 2020, and Biden chose his former rival as his running mate. Their ticket defeated Donald Trump and Mike Pence in November of that year.

Despite suggestions from critics that he was too old to serve a second four-year term, Biden launched a re-election bid in 2023.

Harris plans to go on a book tour of 15 cities, including in the United Kingdom and Canada, for 107 Days. The book is expected to go on sale on 23 September.