A woman found dead in her flat in Bristol could have been lying there undiscovered for up to 18 months.

Believed to be a schizophrenic, the woman's disappearance in 2007 appears to have gone unchecked by housing association officials and mental health workers, despite neighbours raising concerns.

Residents living in Tufton Avenue, Lawrence Weston, named her as Helena Pugh, a mental health patient. Officials have yet to formally identify the body.

Believed to be in her 50s, she was found in the living room of her two-bedroom flat on Monday.

Police are not currently treating the death as suspicious, although a post-mortem examination has so far been unable to determine a cause of death because the body was so badly decomposed.

Sources close to the investigation confirmed she could have died up to 18 months ago.

Neighbours have told how she would regularly refuse to allow care workers and housing association officials into her property and rarely had any visitors.

It is thought she may have a sister, although residents said she had never been seen at the property. Police have yet to track down any next of kin. None of the residents whose flats front onto the same corridor as hers reported noticing a smell or a build up of post.

A neighbour living in the flat below told how he last remembered seeing her on the balcony of her flat, owned by the Guinness Hermitage housing association, in August 2007.

Mature student Nicholas Turner, 47, said: "I first raised concerns with Guinness Hermitage towards the end of 2007, but was told they were unable to gain access to the flat because it was against the law.

"I thought she might have disappeared or have been sectioned – she really wasn't very well.

"That woman should never have been allowed to live on her own. When I last saw her, she looked very ill. She would regularly throw her rubbish in my front garden, shout and swear at the top of her voice whenever she saw people, so I tried to avoid a confrontation by not speaking to her.

"I would often hear her smashing things and on some occasions even heard her growling like a dog.

"She shouldn't have been left to live in the community without someone regularly checking on her.

"She has been let down by the system, which is a shame because if she had been given the right care she may still be alive today."

It is thought a plumber sent round to carry out a check on the gas boiler may have raised the alarm. By law, rented properties need to have a gas safety check carried out every year.

Pensioner Pat Korthals, 72, has lived in the block of flats for more than a decade and also thought her neighbour had been sectioned.

She said: "She was sectioned once before – so I just assumed she been taken away again. When the police told me she had died, I was in complete shock. I can't believe she's been up there all this time.

"I think I last saw her in September or October 2007. I remember her coming down the stairs in her dressing gown. She shouldn't have been living here on her own. You have to ask why she wasn't being regularly checked up on."

Resident Dick Clark, 52, also remembered last seeing her in 2007.

He said: "She was very aggressive towards everyone and to be honest I stayed out of her way as much as possible.

"I thought she had been moved on and didn't really think anything else about it until now.

"This just proves that care in the community doesn't work. No one in the community is qualified to look after people who have such severe mental health difficulties."

Mental health patients living in the community in Bristol often come under the care of the Avon and Wiltshire Mental Health Partnership NHS Trust. A spokesman said he was unable to comment because the body had not yet been formally identified.

Police said they were treating the death as "unexplained".

Wayne Baker, an Avon and Somerset police spokesman, said: "A post mortem examination has been carried out but the cause of death has not been established."

A spokeswoman for Guinness Hermitage housing trust said: "We made repeated visits to the property over the last 18 months to try to make contact with the resident."