Mr Kennedy already had a serious drink problem when he became leader in 1999, according to a new book being serialised in The Times.

But the cover-up began in earnest in 2003, a year after Mr Kennedy sought professional help, when he planned the biggest gamble of his political career by going public with his alcoholism at a press conference in Westminster.

At the last moment he pulled out and his inner circle again refused to admit his problems.

The book reveals that two years earlier, in 2001, senior party figures pressed Mr Kennedy through intermediaries to seek professional help abroad to avoid being recognised.

Fourteen months before the 2005 general election, when the Lib Dem leader’s illness was again hidden from voters, Sir Menzies privately agreed to a request from senior MPs to take over as caretaker if Mr Kennedy was forced to quit.

The revelation in Charles Kennedy: A Tragic Flaw by Greg Hurst, The Times political correspondent. will call into question the judgment of Sir Menzies and other senior Liberal Democrats who suppressed the illness from the electorate. It comes two days after Mr Kennedy indicated that he was ready for a political comeback.

The book details how in 2003 the press conference was planned meticulously. It was scheduled for the first Saturday in July, when Mr Kennedy, with Sir Menzies by his side, would read out a statement confirming that he had a drink problem. Effectively, he would have admitted that he had repeatedly lied when asked in interviews about his drinking.

Sir Menzies was on the early morning train from Edinburgh to London for the press conference when he was telephoned to be told that Mr Kennedy had changed his mind.

In the book Anna Werrin, Mr Kennedy’s secretary and closest aide for more than 20 years, speaks for the first time about his battle with the bottle. “He drank in private, by and large, and drank more than he ought,” she said.

Ms Werrin, referring to the abruptly cancelled press conference, said: “Having slept on it, he [Kennedy] had decided he no longer wished to go through with such a public declaration and would deal with his condition in his own way.”

The statement, which Mr Kennedy was due to read, was quietly destroyed.

Ms Werrin, Lord Razzall, his chief strategist, Lord Newby, his chief of staff, Jackie Rowley, his press secretary, and Sir Menzies were the only key players who knew about Mr Kennedy’s momentous decision to go public in the hope that it would generate sympathy and enable him to carry on in his job as leader of the third party.

“A series of newspaper stories had been published reporting concerns about Kennedy’s recent performances or alluding to his drinking. His advisers feared another was imminent possibly with undignified photographs that risked serious damage to his authority as a political figure. They agreed that he should hold a press conference to set the record straight on years of rumours about his drinking,” the book says.

He would say that he did have a drink problem and was seeking to resolve it.

“It was a pivotal decision. The risks were undoubtedly high . . . but the thought of such a public declaration made Kennedy instinctively recoil. Kennedy continued to battle with his alcoholism in private.”

The book confirms that Mr Kennedy had struggled with a severe alcohol problem throughout his period as a leader from 1999, during which time he fought two general elections with his secret supressed by his aides. “He did not drink excessively every day, but every so often would go on a spree of heavy drinking. He drank gin and tonic, or wine. His hands would shake particularly in the mornings, he would perspire, and exhibit flu-like symptoms.

“At its most acute, his drinking would leave him unfit to perform in public. His staff quickly learnt to throw a protective shield around him and, where possible, keep him completely from view.”

Ms Werrin revealed in the book that she had been worried about his drinking even before he became leader in 1999. During the leadership election in the same year the “symptoms of his alcoholism “were seen by party members”. “But he emerged from the campaign with the full exent of his drinking still largely unknown, assisted by allowing anyone who inquired about his intermittent aroma of alcohol to believe it was due to his choice of aftershave or an alcohol-based mouthwash.”

Within 12 months of becoming leader complaints were coming in about his performances at public engagements, prompting an ultimatum from his advisers that the drinking had to stop.

Ms Werrin said: “It was made very clear to him that you could not combine drinking on anything other than a social basis with being leader. He knew where a complaint had been made. I would help him.”

At a party to mark his 20th anniversary as an MP many of his colleagues walked out in embarrassment because his speech was incoherent. Ms Werrin said: “Yes, it was one of those incidents where he was drunk in public.”

The year before the 2005 general election Sir Menzies and senior MPs met Mr Kennedy, who was asked if he was an alcoholic. He replied: “Yes.” Matthew Taylor, then the chairman of the parliamentary party, replied: “The next time you pick up a drink, you give up being leader.”

Two years later Mr Kennedy was forced out of office because he failed to heed the warning.

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