The River Thames scheme, which will see defence work in Berkshire and Surrey, still has a £45m shortfall.
Consultation is being held in Surrey, Berkshire and London from late August.
Project manager David Murphy said the agency was confident the funding would be found but needed to secure it in the next year.
"Within the next 12 months we need to be certain about where that money's going to be coming from because we've got to put a business case to government," he said.
He said 15,000 homes and businesses were covered by the scheme which would focus on the Lower Thames area from Datchet to Teddington.
Plans include river widening in Elmbridge; work to increase the capacity of weirs at Sunbury, Molesey and Teddington; and the construction of a flood relief channel.
Ideas on how to reduce the cost of a £300m Thames flood alleviation scheme are being sought from the public, the Environment Agency has said.