The temporary car park was installed by Transport for London (TfL) in Wimbledon Park two years ago for use as a depot while carrying out essential works on the adjacent district line - which did not require planning permission.
However after TfL left the site the council did not remove the hardcore and instead submitted an application for a permanent car park arguing there was "excessive demand" for parking and people had been parking on the grass as early as 2008.
It was later decided to remove the structure by the Revelstoke Road entrance and return it to grass, but the work has been delayed by adverse weather recently.
A council spokesperson said: "We have nearly completed dismantling the hard standing and will be restoring the site to a usable state at the earliest opportunity.
"Our work has been unavoidably delayed due to the recent heavy rainfall."
Merton Council had hoped to create a hard surface 52-space permanent car park off Revelstoke Road.