Mr Lewis spoke out as he prepared to publish a book in the New Year which threatens to "lift the lid" on the Government's beleaguered prison policy, sparking fresh embarrassment for the Home Secretary, Michael Howard.
In an outspoken attack on jail policy - just days after prison inspectors walked out of Holloway women's jail over harsh conditions there - Mr Lewis expressed his concern that politicians were overly concerned with excessively tough prison regimes.
Prisons should be austere and there should not be any hint of luxury, Mr Lewis said.
"But what worries me at present is that the political debate - public attention being focused on prisons, the emphasis being given to austerity and security - will upset the balance which is so important.
"Prisons are not serving the public well if they simply turn out embittered, alienated prisoners who were going to go straight back to a life of crime.
"There is a very important rehabilitative task to be done."
He argued that too many offenders "at the margins" were already being sent to prison.
Mr Lewis - who was sacked as director-general in October - also accused Mr Howard of "too much political involvement" in the Prison Service's day-to-day running. He said there was a "very substantial difference of view" between them about what their relationship should be.
Mr Lewis made his criticism just 24 hours after the former Parkhurst Prison governor, John Marriott, branded Mr Howard a "small-minded man" who "mistakes public rhetoric for strong leadership".