NEW DELHI (AFP) - A tell-all book by an ex-aide to India's Prime Minister Manmohan Singh depicting him as a powerless political puppet hit bookstores this weekend, stirring new controversy in the midst of a bitterly fought election.

The book, giving ammunition to opposition critics during the marathon polling which winds up on May 12, paints a damning portrait of a premier who was in office - but "not in power" - while ruling Congress party president Sonia Gandhi called the shots.

Entitled "The Accidental Prime Minister - The Making and Unmaking of Manmohan Singh," the book is by academic and journalist Sanjaya Baru, a former media advisor to the current prime minister, who left his post in 2008.

Traditionally, the president of India's ruling party is also premier. But Ms Gandhi, who led Congress to power in 2004 and 2009, turned down the job, fearing her Italian birth would become an explosive political issue as Hindu nationalists said her foreign origin made her unfit to rule India.

She handpicked Dr Singh for the job but Mr Baru said Ms Gandhi's much hailed "renunciation of power" was more a "political tactic than a response to a higher calling".

He said Dr Singh decided early on to "surrender" to Ms Gandhi and quoted the premier as saying he had "to accept the party president (Gandhi) is the centre of power".

Critics have long charged that Ms Gandhi held the reins of power in the Singh administration but Mr Baru's book is the first by a close advisor to the prime minister to make that claim.

In a strong criticism of the soft-spoken premier, he said Dr Singh "averted his eyes from corruption" to ensure his scandal-tainted government's "longevity".

While Dr Singh, 81, who retires after this election, maintained the "highest standards of probity in public life", he "turned a blind eye to the misdeeds of his ministers".

He thought he could choose cabinet ministers but "he was defanged" as "Sonia nipped that hope in the bud",

Mr Baru said Dr Singh had little authority over his cabinet and a senior bureaucrat would seek Ms Gandhi's "instructions on the important files to be cleared by the PM".

Dr Singh's spokesman Pankaj Pachauri told AFP the book "smacks of fiction" and alleged that Mr Baru misused a "privileged position" for "commercial gain".

The book's release comes as India's multi-phase election is well under way, with the opposition Hindu nationalist Bharatiya Janata Party tipped to win.

Results are due on May 16.