The building is colder inside than out. It is dank and dingy, with corridors covered in dust and detritus, paint peeling from walls, missing ceiling tiles and weeds climbing in through the windows.

This is the Waterlow unit in Highgate Hill, which is the largest single building included in the sell-off plans.

It has been empty and unused for a decade, ever since its previous occupiers, the Camden and Islington mental health trust, made the short move to new premises in Dartmouth Park Hill.

Only the occasional film crew has made use of it – director David Cronenberg shot some of thriller Eastern Promises in the wards.

I am shown around by Philip Ient, the Whittington’s director of estates and facilities, who authored the Estates Strategy.

He said of Waterlow: “This is pretty much as they left it. We don’t use it for anything and never have. It is configured as a mental health hospital and we could not afford to convert it to other uses.”

The building has an area of 4,500 square metres and the hospital says it would cost at least £1,500 per square metre – £6.75million in all – to bring it back into use, as it would need new infrastructure, including lifts, fire alarms, heating, electricity and IT.

Mr Ient said: “We’re talking millions to move in and it just didn’t stack up. In the early days, after the mental health trust moved out, we had to think what to do with it. Could it be used for maternity, for example? It’s not big enough and it’s detached from the main site.

“We came to the conclusion that it was something we would ultimately have to dispose of, but we needed to make sure we looked at the options for the rest of the site before we did that.”

The Whittington has proposed selling off everything north of the road that dissects its land.

There are four other buildings that house offices, staff accommodation, physiotherapy and an education and training centre.

Next up was the nurses’ accommodation, which, with its ageing shared bathrooms and kitchens, was not exactly homely.

Mr Ient insisted the hospital does not want to waste its precious funds on being a landlord.

The listed Jenner building, which houses office staff in its cramped rooms, is deemed unsuitable as the hospital wants to develop “smart working” practices.

Aside from the training centre, which has seen recent investment, bosses say all the buildings are decrepit or unsuited to modern use.

The hospital believes it can raise £17million by selling them off – but only if they go together, because access to the site would be via the Waterlow building.

Mr Ient said: “If we sell it on its own, we put a block on the rest of the site. We see this as a good news story. We are trying to raise money to improve accommodation for patient care. The focus has been that we are selling the family silver, but if part of it is falling down, what use is the family silver?”

Critics have questioned why land should be sold when the hospital rents its Highgate Wing on Dartmouth Park Road. Mr Ient said other options have been explored, such as moving from the Highgate wing into the Waterlow, but the multimillion-pound costs dwarf rent of £244,000 a year.

“If someone comes along with a better way of doing what we’re trying to do, then of course we would reconsider it,” Mr Ient said. “We’re not about to call Foxton’s and say, ‘Put up the for-sale sign’.”

* See photo gallery at the top right hand side of the page for more images inside the Whittington Hospital buildings that could be sold