Advani's comments come two days after the Prime Minister asked the BJP leader not to use harsh words against him.
"The Prime Minister has said that we should not use harsh words. What harsh words have I said? The only thing that I have said is that among all the Prime Ministers I have seen since (Jawaharlal) Nehru, Manmohan Singh is the weakest. This is a political comment. Where is the harshness?'' Advani told a press conference.
Making it clear that he will not retract his comments, he said, "The Prime Minister claims he is hurt because I called him the weakest Prime Minister in living memory. If calling a spade a spade is improper, I plead guilty. But it is not me, it is the Supreme Court that has observed that the 2G scam could have been avoided and the estimated Rs 1.76 lakh crore loss to the exchequer saved had Mr Manmohan Singh acted".
Advani, who was equally unsparing about the Congress, said, "With every passing day, the UPA government, nominally led by Manmohan Singh, but commanded by Sonia Gandhi, is exhibiting all symptoms of terminal decline.
"Several of its former ministers are crowding the cells of Tihar Jail on charges of corruption, others are waiting in the queue to enter. Senior ministers of the PM's own party are publicly criticising and contradicting one another," Advani said without elaborating.
"The ruling party's most powerful general secretary makes daily statements that are routinely contradicted by other leaders of the party. Such a condition in a patient would be described as multi-organ failure," he said. .