Overcrowded primary set to add even more pupils
The parent council claims the plan would leave the school with "woefully inadequate accommodation" for the foreseeable future as it already struggles with the limited dining and gym facilities it has.
Education bosses agreed to try to ease the accommodation pressures at the popular school as part of the council budget last year.
The original plan was to provide five extra classrooms to ease pressure on general purpose space and move the nursery from a dilapidated annexe into the main school.
But parents say the council is abandoning the "principal rationale" for the extension by instead trying to cram eight extra classrooms into the Victorian school to cope with a rising roll, rather than deal with the problems the school is already facing.
It would also mean leaving the nursery in the annexe, which has been deemed unfit and has a leaking roof.
Sean Watters, chair of the school's parent council, said: "The extension was never intended to increase the capacity of the school, its purpose was to address some, though not all, of the deficiencies in the accommodation and in particular the split site.
"If anything, the capacity would actually be reduced.
"The whole reason for the project was to get rid of the annexe.
The annexe is in a shocking state."
He added: "As of this August we have got no general purpose space as we have lost our library and music rooms to classrooms."
Parents say that the council had previously agreed to limit the number of classes to 18 because it admitted the school could not cope with the additional strain.
A report which went to the education committee at the end of last year said: "It has been established that the school should in future continue to operate at 18 classes, as to extend the building beyond this capacity would prove problematic given the physical nature of the building and the constrained site."
This is the latest in a long line of setbacks for the project, which was approved in 2009 along with an extension to Corstorphine Primary at a total cost of 4.1 million.
Work to increase the accommodation at Towerbank was originally due to start this year, with a completion date scheduled for 2011, but council bosses revealed at the end of last year that it would be delayed.
Completion was then scheduled for August 2012, but has once again been delayed and parents have now been told it won't be finished until February 2013.
They have called on education bosses to have a thorough consultation with the school before going ahead with the new plans.
City education leader Councillor Marilyne MacLaren said: "I have spoken at length with Mr Watters and the headteacher and we agreed that work will be continued over the summer to look at all the options.
"We will return to discuss the options and we will try to come up with a way forward that is agreeable to everyone."