Emeritus Senior Minister Goh Chok Tong yesterday said he was saddened to see how his long-time friend Tan Cheng Bock has "lost his way".
Mr Goh's criticism of his former party colleague and now political opponent, made in a Facebook post, came a day after Dr Tan officially launched his Progress Singapore Party (PSP).
In his first public comments on Dr Tan since the PSP was formed, Mr Goh said: "Tan Cheng Bock was my classmate in Raffles Institution. I have known him close for over 60 years. It saddens me to see how he has 'lost his way'."
"He is like Don Quixote tilting at windmills," he added, using an expression from the Spanish novel by Miguel de Cervantes that means to attack imaginary enemies.
At a press conference last month, Dr Tan had criticised the People's Action Party (PAP) for an "erosion of transparency, independence and accountability".
During his party launch on Saturday, the former PAP MP said he had decided to re-enter politics "for country, for people". He also recounted how founding prime minister Lee Kuan Yew had asked him to join the PAP, adding that it was now his turn to ask Singaporeans to join him.
To this, Mr Goh wrote: "Ouch! (Dr Tan) omits to say that I put his name up to LKY. Surely I deserve some credit - or rather, blame - for who he has become now? 'For Country, For People'. He has conveniently left out 'For Me'!"
His latest post is arguably the harshest Mr Goh has been towards Dr Tan, who was one of three visitors Mr Goh allowed apart from immediate family when he underwent surgery for prostate cancer in 2014.
Dr Tan, who narrowly lost in the 2011 presidential election, had announced his second bid for the elected presidency in March 2016 while a Constitutional Commission was reviewing the process.
At that time, Mr Goh said Dr Tan's decision was a "calculated political gambit" that could be seen as politicising the process. He also said: "We are still very good friends, still go out with each other, but I will not try to influence him... I will just wish him good luck."
Dr Tan, who was the PAP MP for the former Ayer Rajah single-seat constituency for 26 years, is making a bid to re-enter politics at the next general election, which must be held by April 2021.
Asked about Dr Tan at a National Day dinner in Joo Chiat yesterday, Senior Minister Teo Chee Hean said: "The 2G have helped the 3G people like us to find our feet and to bring Singapore forward for the last 15 years. We should really be looking forward to the 4G... We shouldn't be going back to the 2G."