ELEVEN people remained homeless today after mystery carbon monoxide fumes forced the evacuation of Derby flats.

Saturday's incident was the second time the lethal gas has hospitalised tenants at Pride Park Lodge, Meadow Lane,Chaddesden, in just over a year.

At an emergency planning meeting held yesterday, landlord Steve Naylor was advised that if carbon monoxide detectors were fitted in all flats, communal areas and stairways, that residents could return to their homes.

But Mr Naylor decided this would not be appropriate until a full investigation was carried out to find the source of the gas.

On the first occasion, in November 2008, an investigation concluded the fumes were probably caused by a resident pouring chemicals down a drain but the reason for the latest incident remains a mystery.

Yesterday's meeting included people from Derby City Council and Derbyshire County Council, who operate the county's emergency planning team, Derbyshire Fire and Rescue Service, Derbyshire Police, the Health Protection Agency, and a representative for Mr Naylor.

A city council spokeswoman said the landlord had volunteered not to use the building and would be finding new accommodation for tenants with the help of his letting agent, IMS. She said there were no records to suggest rumours that the flats, built in 2005, had been constructed on a former landfill site were true.

But she said the source of the carbon monoxide was likely to be underground, as the building had no "fossil-fuel burning sources" like gas central heating.

She said: "This is extremely unusual but not an unknown occurrence and at this stage we do not know what caused this more recent incident and further investigation will be needed to identify the cause."

Three evacuated tenants have been put up by the city council in bed and breakfast accommodation, seven have found lodgings with family and friends and one is on holiday but has been in contact with the landlord.

The spokeswoman said there were no children or elderly people living in the flats and that no date had been given for when people could live there again.

On Saturday, four residents were taken to hospital and later discharged after inhaling the poisonous gas.

The worst affected was Tony Richardson, 31, who said he felt sick and dizzy.

Mr Richardson said: "I don't want to be living there because it's unsafe. I'm happy with the Travelodge the council has put me up in."

In 2008, a couple and their baby were hospitalised.

Mr Naylor said he was not yet sure what the investigation into the gas would involve and that he had been focusing on finding accommodation for the tenants.