They include diver Tom Daley, fresh from his victory at the World Championships last week, visiting the Olympic Park where he hopes to go for gold in London 2012.
Daley, 15, is among a squad of British Olympic and Paralympic stars and dignitaries marking the three-year countdown to the Games with a special trip to the Olympic Park in Stratford.
It is a big moment for Olympic organisers who stress that despite the biting global recession, the £9.3 billion Olympic project is on target and on budget.
International Olympic Committee (IOC) president Jacques Rogge has given the thumbs up to London 2012’s progress so far.
He said: “A little over four years ago, Seb Coe [London 2012 chairman] presented a vision to the IOC members of Games that would make a difference.
“Make a difference to the athletes by giving them the experience of a lifetime, make a difference to the lives of people in London and across Great Britain through regeneration and sport, and make a difference to the IOC and the Olympic Movement.
“London 2012 and its partners are delivering on that vision.”
The visiting athletes will travel on the Javelin train from St Pancras, on the 140 miles per hour high-speed line, to Stratford International Station – a service which will bring up to 25,000 visitors per hour to and from the Olympic Park in 2012.
Coe said: “In exactly three years, London and the whole of the UK will be getting ready to stage the greatest event on earth and we will not disappoint.
“The athletes who are visiting the park will get a small glimpse of the sporting stage that they – and we – have spent the last few years working towards.
“There is a tangible sense of excitement building and a genuine sense of belief that the Games in 2012 will not only be a source of intense national pride, but that the benefits will be felt for generations to come.
“We are exactly where we would want to be at this moment.
“Our overarching message is that we’re halfway through on our journey.”
Olympics Minister Tessa Jowell repeated her claim that London staging the Games is helping boost the British economy during one of the worst economic crisis’ in history.
She said: “Some people have asked me if we can afford to have the Olympic Games in mid-recession.
“We cannot do without them.
“They are a shot in the arm for the economy, through construction contracts for mainly-UK firms while creating jobs and training for new skills.
“This truly is a once-in-a-lifetime opportunity.
“We’re on budget, we’re on track, and we have an ambition to deliver a great Games.”