The Public Bodies Bill, which will pave the way for selling attractions such as the New Forest and Sherwood Forest, had its second reading in the House of Lords this week.
Lord Clark, who stood down as chairman after eight years in January, said: "It is a nonsensical move that won't save any money. The land is very attractive because it is free from inheritance tax.
"The Government will take the money now but will lose out because it will not get the inheritance tax. For pension funds there are tax advantages, too."
Lord Clark said the Government's value of £5bn for the forests was wrong - he put the value at £0.5bn. He added that biking and horse riding trails could be closed: "This will certainly will mean less public access. It will be disastrous for the Lake District, for instance. I think it is a big risk. Annually, it costs just 30p per person per year to have the forest estate."
Lord Clark said: "It depends on the whim of ministers - if they get the bill through. There are all sorts legal problems in selling it. That's why they want the bill."
He added that he expected "widespread opposition" to the bill and sell-off, which could start as early as next spring.
"They will have the power to do anything. The worst case scenario is they sell everything off and the public has no access whatsoever - and there's the threat to biodiversity that goes with that," he said.
Public access to the sites is currently provided by a dedication under section 16 of the Countryside & Rights of Way Act 2000.
Well-known forests such as Kielder, Bedgebury and Westonbirt arboreta, Cannock Chase and the Forest of Dean, Whinlatter and Grizedale are all under threat of being sold.