PPPS chairman Peter Walker said the society's investigations into legal documents dating back to the 1920s revealed land designated for public open spaces extends well beyond the park fence.
He added: "One key area is a large shrubbery between the current railway bridge and Lookers car sales showroom.
"This is where the council want to build a new bridge over the railway, but this area currently has 29 mature oak trees as well as 32 other trees. Most of these were planted in the 1920s within weeks of the transfer of land from RA Jones' estate to the Southend Corporation."
Although the plans have yet to be formally agreed and the scheme in principle is currently open to public consultation, PPPS chiefs are worried the trees could be felled to allow a widened road through.
Mr Walker explained that although the previous scheme had been booted out because it was unacceptable, he felt councillors could not see anything wrong with the latest idea.
He added: "What they must realise is it is simply not possible to widen Priory Crescent without using park land because the land on both sides of the road was included in the terms of transfer in the 1920s.
"These councillors will be taking a decision in the next few months which could alter dramatically the character of Southend."
Roger Weaver, Southend's executive councillor for engineering, planning and transportation, was contacted for a comment, but he was unavailable at the time the YA went to press.