London Fire Brigade was called to Royal Free hospital on Tuesday after staff said they smelled something unusual' - the third time in eight days a strange smell has been reported at the Hampstead hospital.

Nobody was taken ill on Tuesday and it was concluded the smell on that occasion was caused by what the hospital called solvent-based products' which are being used in refurbishment work taking place on the hospital's first floor. But the hospital is not connecting Tuesday's incident with two other instances last week when staff were taken ill because of what the hospital described again as an unusual smell', though not the same odour.

A hospital spokeswoman said investigations are on-going as to what last week's smell was or what caused it, and there is no agreement as to what the stench could be compared to. London Fire Brigade described it as fumes', while the hospital spokeswoman said some people had said it was a sweet smell' similar to pear drops.

The hospital said in a statement: "Extensive investigations of the air conditioning system, the drainage system and the cleaning chemicals used in the building, none of which have changed in the past few years, have failed to establish a cause.

"The hospital has resumed normal activities. The Health and Safety Executive has been asked to lead the continuing investigation and it will work closely with the Health Protection Agency."

Last Monday, 11 members of hospital staff were taken ill because of the smell. Then on Wednesday another eight reported feeling unwell after more unusual smells. That day ambulances bringing in emergencies were redirected to other hospitals, though contrary to reports, A&E remained open.

None of the staff who reported feeling unwell were seriously ill and all have now returned to work.

Arthur Brill, chairman of the Patient and Public Involvement Forum for the Royal Free, said services had not been affected, adding: "Let's hope it doesn't recur."