Britain's blood stocks need to rise 30% above normal levels in time for the Queen's Jubilee and the Olympic Games this summer, according to the NHS.
The NHS has warned that regular donors miss appointments more regularly when large public events take place because normal routines are suspended and people travel more frequently.
The influx of visitors from overseas will also increase medical need and may place pressure on certain blood groups that are more common in other countries.
Britons with O positive and O negative blood types are particularly in demand, with additional appointments being created and some facilities extending opening hours to give donors more flexibility.
First-time donors are also being encouraged to give blood to cope with the demand.
One problem for the health service is that blood decomposes very quickly, with red blood cells and plasma lasting just 35 and seven days respectively.
As a result, supplies have to be replenished frequently to ensure a steady flow for medical use.
Hospitals in England and North Wales need 7,000 units of blood every day and each unit saves or improves the lives of three patients.
Jon Latham, spokesman for NHS Blood and Transplant, said: "This year we have a unique situation and need the public's help to counterbalance the inevitable dip in donations as people celebrate a memorable summer for the country. We need to adjust the mix of stocks so we are well prepared as a responsible host for these exciting events."