Experiments growing two hazardous fungi, Histoplasma capsulatum and Paracoccidioides brasilienis were taking place in a 'category 3' secure laboratory at the United Medical and Dental Schools, Guy's Hospital, when flasks containing the fungi in an incubator broke for no apparent reason, a spokeswoman said.
Twenty members of the laboratory staff who had been in the vicinity were given blood tests but cleared of infection after the accident in September. They have remained well.
No further research on the fungi, which can cause lung problems, has been carried out since the executive has ordered hospital managers to improve the ventilation system in the laboratory.
The incident is the second of its kind to come to light this month. It follows the imposition of restrictions on Birmingham University after questions were raised over the safety of genetic experiments involving viruses similar to that which causes the common cold.
Cancer-causing genes were being put into a virus in circumstances the executive said might not be adequate.