The Scottish Prison Service (SPS) has confirmed that preparatory work, including the removal of trees, is under way at the former secondary school site off Inverkip Road.
Contractors are expected to move in soon and flatten the building, pictured, paving the way for the new £60m HMP Inverclyde female-only jail.
The nearby former Glenburn School will also be knocked down as part of the site clearance. Prison chiefs expect the buildings to be completely gone by the end of the year.
It is understood that once the preparatory works are finished jail bosses will put the large demolition contract out to tender.
A spokeswoman for the SPS said: “Demolition works have not started.
“We are doing preparation works for the site, including clearing trees.
“If all goes to plan, the demolition will be completed by the end of the year.”
The SPS bought the nine-hectare site from Inverclyde Council in 2011 before leasing it back to the local authority.
For decades it was home to Greenock High and was most recently used as a decant building for St Columba’s while a new school was built for its pupils and staff in Gourock.
The Tele first revealed in 2010 that the Inverkip Road site was to be used for a new jail.
Bosses had initially planned to build a replacement for the 100-year-old Gateside prison there but those plans were shelved. It was announced in 2012 that the new jail would be for women only, replacing Cornton Vale, near Stirling.
Prison officials also confirmed that Gateside would also remain open, meaning Greenock is to become a two-prison town.
A date for the start of construction work and for the opening of the new jail is not yet known but it was initially hoped that it would be operational by 2016.