"I think smoking is a scourge and it's right to discourage it (but) I am very sceptical at the moment."
He drew on personal experience as he described his opposition: "I have to think back to my own life two decades ago when my wife and I had a baby.
"It came to that point when everybody was asleep and I was in such a mood of absolute elation I wondered out into a park in Islington and it was in the middle of winter but I laid on the ground and had a cigar.
"I don't want to be in a city where somebody can stand over me and say you've got to pay £115 for doing something that is of no harm to anybody except me."
He insisted there is "a great deal in this superb report that we can take forward".
There appeared to be no appetite from the Government to roll out a ban on smoking in parks nationally.
Asked whether David Cameron would back a ban on smoking in public parks, the Prime Minister's official spokesman told a regular Westminster media briefing: "The Government has no plans for that."
Local government minister Kris Hopkins said: "All local taxpayers, including smokers, should be able to enjoy the use of municipal parks, provided they show social responsibility towards others."
Lord Darzi said he respected the Mayor's opinion but insisted he believed smoke-free parks would become a reality.
He said: "We have started a debate today. I think this debate will continue.
"I have no doubt in London in due course, certainly in my lifetime, the parks will be smoke-free.
"I am a cancer surgeon and I have seen the impact of smoke in cancer and I would like our parks to be the beacon of health in London."
Antismoking campaign group ASH welcomed the report but Simon Clarke of pro-smoking group Forest said a ban would be "outrageous".
In a blog post, he wrote: "There's no health risk to anyone other than the smoker. If you don't like the smell, walk away.
"Tobacco is a legal product. The next thing you know, we'll be banned from smoking in our own gardens in case a whiff of smoke travels over the fence."
Professor Robert West, director of tobacco studies at University College London, said the plan "seems draconian" and "tests the limits of how far it is reasonable to limit the freedoms of some members of society in what is seen as a good cause".
But he said it could save lives.
"If it helped as few as 100 smokers a year to stop who would otherwise have carried on, that would amount to some 50 human lives saved a year," he said.
Prof West said the success of the potential measures would hinge on the approach of smokers.
"If smokers overwhelmingly support such a ban, having heard all the arguments, then it seems reasonable to put it into place. If they don't, then it probably is not and in any event may not be enforceable without their consent."
Professor John Britton, who leads the tobacco advisory group for the Royal College of Physicians, said that if London takes up the ban, councils up and down the country would quickly follow like "dominoes".
He said: "The ban is a very good idea. Prohibiting smoking in a few places has a symbolic effect. It is a small step, but it is the kind of thing that, if rolled out nationally, could have a big impact.
"I know there are other councils thinking about the same measures."
He said Nottingham City Council said last month it would extend its successful no-smoking policy for children's playgrounds to other public places where the community wants it, and other local authorities are said to be considering similar measures.
Prof Britton said: "Unquestionably, this would have an effect across the country. If someone takes the plunge, we will see a domino effect. It could save lives."
The report has further recommendations to improve health, including minimum pricing for alcohol, traffic-light labelling on restaurant menus, restrictions on "junk food outlets" near schools, Oyster card discounts for people who walk part of the way to work, and measures to reduce air pollution.
It also calls for a £1 billion investment to modernise GP surgeries, one third of which the report found to be "very poor" or "unacceptable".
Other measures in the Better Health For London report include selling off unused NHS land and giving new mothers control of some of the payment for their care.