Blood supplies of the O positive type have dwindled, the National Blood Transfusion Centre said on Friday.
The warning came just a day after the centre said supplies are not coping with the number of ill people.
The centre said there was an "urgent" need for blood of all types amid insufficient donors to keep up with the demand.
It appealed for caution from drivers, especially during the rain, to avoid more traffic accidents which put pressure on the current situation.
The blood transfusion centre, located next to St Luke's Hospital, is open daily between 8am and 6pm, including public holidays. Donors should provide their ID card or driving licence to donate blood.