Priory Park Preservation Society (PPPS) chairman Peter Walker claims the controversial scheme, which will cost acres of protected open land and 120 trees, is liable to become the subject of a judicial review.
Mr Walker said the action group was left with no alternative to a legal challenge after the town hall Cabinet approved the plans to widen the road at a meeting on March 5.
He added: "When our solicitor read the report, he wrote to the council highlighting 15 major legal shortcomings.
The project has been the subject of much debate and protest action from various groups in the borough.
The majority of residents who have objected to the £4 million road building plans are against it not just because they claim it is environmentally damaging, but, Mr Walker claims, because it will not do anything to solve the borough's traffic problems.
Mr Walker says the PPPS was asking its members and residents against the scheme to help fund the legal action.
He added: "We pay our council tax only to see it squandered by an incompetent local authority and then we have to pay again for a solicitor and possibly a barrister to make them do their jobs properly. We are looking at a sum of at least £10,000."
At the cabinet meeting on March 5, councillor Roger Weaver, cabiner member in charge of transport, pledged that no trees would be lost from the park's grounds.
The council also agreed to replant any trees lost on a two to one basis when the dualing scheme was finished.
Residents wishing to controbute to the PPPS fighting fund should send their contributions to Priory Park Appeal, 62 East Street, Southend, SS2 6LH.
[Note that cheques should be made payable to "The Priory Park Preservation Society"].