Viridor is preparing its planning application for the facility, which is expected to be submitted in May.
It is hoped public consultation on the proposals could begin as early as mid-February.
The leaked document reveals achieving planning permission is vital for the project - and the plan could still be scuppered by Sutton Council planners, as well as the Greater London Authority (GLA).
Beddington Lane is on Metropolitan Open Land, (MOL) requiring the GLA to be consulted on planning matters.
The document admits Viridor’s proposal, while consistent with EU regulations, is contrary to MOL conservation policies and must be proven to be “very special circumstances” to receive planning permission.
The incinerator is to be built partly on land which is intended to form part of the proposed 8,000 acre Wandle Valley regional park.
It is understood under Sutton Council’s planning blueprint the area should be part of the park in 2023, which would not be the case if the incinerator is built.
The SLWP document states should the plan stall, through delayed planning permission, or construction delays, there is a great risk of escalating costs, that could negate any savings from snubbing the WRG bid.
Coun Thomas, said: “Viridor's proposal will be subject to a detailed planning process, which will consider issues around land use, design, traffic, air quality and sustainability.
“A significant part of this will also be the impact on the country park, which incidentally, Viridor fully support. This proposal is to occupy only a tiny fraction of the country park (around three per cent).
GLA member for Croydon and Sutton Steve O’Connell, who is also a Conservative Croydon Councillor, said: “It is too early to comment.
“I don’t know about the plans or any health implications. Clearly we need to look at the impact on local people and need to look at the application once it has been finalised. Until that time it is too early to comment."