The service, which provides first aid to thousands of sports, arts and other public events every year, has launched a recruitment campaign aimed at anyone with time on their hands, including mothers wanting to return to work.
There was an 8 per cent fall in adult volunteers in 1997 to 57,525 and a 14 per cent decline among cadets.
Peter Carver, the Commissioner in Chief, said: "The future is uncertain if the charity continues to lose volunteers at the current rate.
"The losses experienced year on year are unsustainable. It is absolutely vital that we attract new volunteers. The only qualification needed is a little spare time."
Unpaid St John volunteers give more than half a million hours of care at public events every year, treating 150,000 casualties in London alone.
Major events are required by law to have first aid experts on hand and some groups, such as football clubs, pay a small fee to the charity. Several volunteers are on duty at the Albert Hall every night of the year and 1,400 attend the London Marathon.
The recruitment campaign coincides with the 900th anniversary of the Order of St John, which was established by a group of monks from the Abbey of St Mary in Jerusalem who set up a hospital to care for sick pilgrims.
The Order prospered in Britain until the reign of Henry VII who confiscated the land and property that had been donated to the service. It was revived in England in the 19th century and now has 300,000 members in 42 countries.
Volunteers are needed to drive cars and ambulances, cook, raise funds, work in charity shops or help the growing social care programme.
Anyone interested can call a special hotline - 0870 1290900.