The BJP also said that on the eve of relinquishing the office Singh must "seriously introspect" as to how his tenure "impacted" on the institution of prime minister.
Party patriarch L K Advani led the BJP charge as he referred to his 2009 Lok Sabha campaign in which he had harped about Singh being the "weakest" ever PM and said the book written by Sanjaya Baru confirms what the world already knew.
"The world already knows what he (Baru) has said but the book is an official confirmation. When I said for the first time that among all our PMs he (Singh) is the weakest, then my own colleagues said he is a nice man and why criticise him so much. I said I feel sad and had sympathy for him," Advani told reporters in Ahmedabad.
Advani was asked to comment on the book titled "The Accidental Prime Minister: The Making and Unmaking of Manmohan Singh".
Leader of Opposition in Rajya Sabha Arun Jaitley said that among various institutions which have been "dwarfed" in the UPA regime, the "principal one" was the office of the Prime Minister itself.
Taking a dig at Singh, he said Prime Ministership is not an employment but a public service and an exercise by which leadership is provided to the people.
"On the eve of relinquishing the office, Dr. Manmohan Singh must seriously introspect as to how his tenure impacted on the institution of Prime Minister," he wrote on his blog.
Jaitley sought to draw a parallel between the UPA government and old communist states, wondering if Congress President Sonia Gandhi was like the general secretary of the communist party who had the last word and not Prime Minister Manmohan Singh.
Quoting the book, he said it confirms what the country "suspected" that the PM had to get most of his decisions approved and ratified from the Congress President and all sensitive subjects had to be discussed with the person outside the government.
"Did he (PM) have the last word on all subjects Or is it the system of the original communist states that was operating, where the party general secretary was always more important than the head of the government," he said.
"The appointments of key officials would be regulated by 10 Janpath (Gandhi's residence). Contracts such as the coal blocks allocations were allotted by the party. Even the venue for the funeral of former Prime Minister P V Narsimha Rao was decided on collateral considerations....," he added.