Thousands of events are being held in the coming weeks with the aim of attracting people to do voluntary work to mark the first anniversary of the London 2012 Olympic and Paralympic Games.
The legacy charity for volunteering, Join In, said it hoped 100,000 new volunteers would come forward between now and early September to offer help in their local community.
Research has shown that most sports clubs need more volunteers to meet a surge in demand in participation as a result of the Games, but people do not know how to offer their services.
Alice Hunt, chief executive of Join In, said: "The great surprise of the London 2012 Games was the success of the volunteers but thousands have told us they don't know where to go to find out about opportunities to carry on volunteering in their local community.
"Sports clubs and community groups are crying out for help and so at Join In we're match-making people willing to give up their time to the organisations who need them most. This summer we will unleash an army of volunteers to enable the next generation of sporting heroes to flourish and make a difference to communities across the country."
Sebastian Coe, the Prime Minister's Olympic and Paralympic Legacy Ambassador, said: "It took us 10 years to deliver the Games; it will take 10 years to deliver the full London 2012 legacy. Last year the Games Makers, along with all the volunteers in London and around the UK, made the Games the success they were. Most importantly they inspired people - showing what it means to be a volunteer, in both attitude and impact."
Meanwhile, the Royal Mail has confirmed that the 111 post boxes painted gold to mark the success of Britain's gold medal winners, will remain permanently in that colour.
The boxes, which the Royal Mail said had become "treasured local landmarks", have received a fresh coat of paint.