Drawing on a trove of private papers from Hillary Clinton’s best friend, the legendary Watergate journalist Carl Bernstein is to publish a hard-hitting and intimate portrait of the 2008 presidential candidate, which will reveal a number of “discrepancies” in her official story.

Bernstein, who was played by Dustin Hoffman in the film All the President’s Men, has spent eight years researching the unauthorised 640-page biography, A Woman in Charge: The Life of Hillary Rodham Clinton.

“Bernstein reaches conclusions that stand in opposition to what Senator Clinton has said in the past and has written in the past,” said Paul Bogaards, a spokesman for Knopf, which publishes the book on June 19.

With the thoroughness for which he is famous, Bernstein spoke to more than 200 of Clinton’s friends, colleagues and adversaries. He stops short of accusing the New York senator of blatantly lying about her past, but has unearthed examples of where she has played fast and loose with the facts about her “personal and political life”, according to Knopf.

The book could revive the explosive charge, made earlier this year by David Geffen, a former Clinton donor and Hollywood mogul, that “the Clintons lie with such ease, it’s troubling”.

Clinton remains the frontrun-ner for the Democratic presidential nomination, but Barack Obama, who is keeping pace with her fundraising juggernaut, is closing the gap in the polls.

The Sunday Times has learnt that Bernstein has been given unprecedented access to the private papers of Diane Blair, Clinton’s closest friend and confidante, who died of lung cancer aged 61 in 2000. The collection is still being sorted at the University of Arkansas library and is not yet available to the public.

Bernstein has been delving through Blair’s copious records of the 1992 presidential election campaign, which could offer tantalising insight into Bill Clinton’s war machine and Hillary’s reaction to news of her husband’s dalliance with the nightclub singer Gennifer Flowers in Arkansas.

Hillary denied all knowledge of the affair, but one writer who has followed her career closely said: “She always knew about her.” He added: “Anyone who has approached the subject of Hillary Clinton with a clear eye will run across many examples of stories that are not true.”

Blair, a professor of political science, crisscrossed the country with the Clintons in 1992, serving as a senior adviser and semiofficial historian of the campaign. She became friends with Hillary in the political backwater of Little Rock, Arkansas, in the 1970s, when the two East Coast-educated power women sought each other out as soulmates.

Hillary went on to serve as “best person” at her friend’s marriage to Jim Blair, who had a walk-on part in the scandals of the Clinton White House when it emerged he had helped the former first lady make $100,000 in cattle futures.

Joe Klein, the bestselling author of Primary Colors, recounted how Blair once witnessed a blazing row between Bill and Hillary Clinton. “They were really, really angry with each other,” she told him. “And then suddenly, the president took her in his arms and began kissing her all over her face and he said, ‘God, what would I do without you?’ I felt kind of embarrassed being there.”

When Blair was diagnosed with lung cancer, Clinton was running for the Senate in New York. In her memoir, Living History, she writes about seeing her friend for the last time in an Arkansas hospice. “She pressed my hand tightly and whispered to me, ‘Don’t ever give up on yourself and what you believe in. Take care of Bill and Chelsea. They need you. And win this election for me’.”

Bernstein is known as a liberal Democrat who fiercely opposes the war in Iraq and is likely to be critical of Clinton’s Senate vote to authorise the war. His marriage to Nora Ephron, the screen-writer, broke up when he had an affair with Baroness Jay, the daughter of former prime minister James Callaghan.

For years Bernstein suffered from writer’s block, but Knopf is promoting his biography as a triumphant return to form. Publisher Sonny Mehta said his portrait would “show us, for the first time, the true trajectory of Hillary Clinton’s life and career”. It will be published simultaneously in Britain by Hutchinson.

According to the publishers, it will cover everything from Clinton’s “complex relationship with her disciplinarian father” to “her courtship with Bill Clinton and the amazing dynamic of their marriage, during the most trying of circumstances”.

Clinton’s relationship with the truth has frequently come under scrutiny. William Safire, a conservative columnist in The New York Times, provoked a storm in the 1990s when he accused the first lady of being a “congenital liar”. Bill Clinton let it be known that if he were not president, he would punch Safire on the nose.

While the senator continues to lead Barack Obama, her nearest rival, in the polls - most recently by 36% to 31% in an NBC/Wall St Journal survey - she continues to be dogged by high ratings for “unfavourability”.

She is the most assured Democratic candidate on the campaign trail, as she proved in a televised debate with her rivals for president in South Carolina last week. But while Washington commentators declared her the obvious victor, television viewers in the state put Obama on top, suggesting there remains considerable voter-resistance to her charms.

In an effort to boost her campaign, Clinton said last week that she would appoint her husband ambassador to the world if elected to the White House. “I can’t think of a better cheer-leader for America than Bill Clinton, can you?” she said.

Bernstein’s biography is likely to touch some raw nerves. One writer who has crossed swords with Clinton advises Bernstein to watch his back. “She has the most powerful war machine that has ever been developed and it is led by people who have been to hell and back.”

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