According to the bank sources, the mishap happened during a training programme of 150 officials from different commercial banks. It was held on the 24th floor of the Bangladesh Bank headquarters in Motijheel.

At around 11 in the morning, each of a group of 50 trainees was served with a sandwich and a banana. About two hours after taking the meal they began complaining of an upset stomach and were immediately taken to the medical centre on the bank premises.

As condition of 21 people worsened they were immediately shifted to nearby Islamic Bank Hospital in Motijheel.

Dr Shafiqul Islam, emergency medical officer there, diagnosed 18 of them as suffering from food poisoning.

"We kept four including one female patients under observation here in Motijheel hospital. Because of shortage of bed we had to shift more stable ones to our branch in Kakrail," said Dr Islam.

However, by 5:00pm all the patients were released as their condition improved, the hospital authorities said.

"Luckily, none of the patients got into critical condition. As they managed to throw up the food, the poison couldn't reach the blood stream and didn't do any harm," Dr Islam added.

Meanwhile, Governor of Bangladesh Bank Dr Fakruddin Ahmed visited the sick officials in the hospital yesterday afternoon. He instructed officials concerned to initiate an investigation and take necessary action.

Stale sandwiches supplied by a fast food shop are believed to have caused the contamination, sources said. No one was held in this connection until 7:00pm.