The Hoxton flat has no heating or hot water and the kitchen floor was under three inches of dirty water because of a leak.
Water and damp has rotted the woodwork, causing mould to spread through every room in the property.
Mould covers all of the furniture, the toilet is broken and the kitchen cupboards have rotted because of the leak.
The resident's carer said they started complaining about repairs two years ago, but did not receive a visit.
Housing specialists Liverpool-based High Street Solicitors have now taken on the case and hired an expert to access the property.
"It’s difficult to tell how far the damp and water has spread but all floors are saturated, the electrics remain in use," the independent expert said in their report.
"They need to be turned off and condemned due to potential risk of electrocution.”
After being contacted by the Standard, Hackney council said it had moved the resident into temporary accommodation and has now launched an investigation into how the flat became so dilapidated.
Larissa Ellis, head of housing at High Street Solicitors, told the Standard: "This is the worst case I have ever seen. The flat is completely uninhabitable.
"You can tell [the resident] is vulnerable. The property is clearly affecting his mental health, he seems very anxious about the whole situation.
"Your home should be a safe and comfortable space. It is far from that and it's clear there is now a real risk to his life.
“There were no visits from the council despite multiple complaints for two years. Only when we were instructed and had a professional survey the property and produce a report did someone from the council visit the property.
“It is unbelievable that it has got to this stage and we are doing everything we can to help him."
It added that the flat and repairs are managed by the council-funded Wenlock Barn Tenant Management Organisation (TMO).
The local authority said that during pandemic lockdowns the TMO only undertook repairs where there was a risk to residents or a possibility of serious damage to their property, which has resulted in a backlog.
A council spokesman added: “This property is managed by Wenlock Barn Tenant Management Organisation. According to their records, the last reported repair was on January 23 2020 - a repair to a hand wash basin that was completed.
“We are very sorry that this resident has been living in conditions like these. We have supported him to move into temporary accommodation, and are working with him and his carer to assess whether he needs more support.
“We have carried out an initial inspection of the property in order to assess what further work is needed.
“We are also investigating how this property came into such disrepair, and will be working with the TMO to support them to take appropriate action to resolve these issues."
He added the council is still suffering the effects of a cyberattack in October 2020, which hit repair service systems.
Wenlock Barn Tenant Management Organisation have been contacted for comment.
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