Organisers have vowed to produce the biggest, "most exuberant" river pageant ever seen on the Thames
Celebrations for Queen Elizabeth II's diamond jubilee reach their climax on June 3 when a flotilla of 1,000 boats sails up the River Thames in a spectacle that presents formidable security challenges.
Venetian gondolas and a Chinese dragon boat will join kayaks, motorboats and a 19th century French trading ship for the first pageant on this scale on London's river since 1662.
Organisers have vowed to produce the biggest, "most exuberant" river pageant ever seen on the Thames -- but they admit the logistics of the Â£10.5 million ($16.5 million, 13.1 million euro) event will be "extraordinarily complex".
With the queen's ceremonial barge carrying the royal family and more than a million spectators expected to line riverbanks and bridges, the procession will double up as a security rehearsal for the London Olympics that open on July 27.
Heir to the throne Prince Charles and his wife Camilla, as well as Prince William, his wife Catherine and his brother Prince Harry, are joining the queen and her husband Prince Philip on the sumptuous red and gold royal barge.
Music ranging from the national anthem and chiming bells, to Bollywood tunes and the famous James Bond theme will blare from boats as the 11-kilometre (seven-mile) long flotilla makes its three-hour journey.
The Thames flood barrier will be closed to ensure calm waters and traffic will be barred from seven of London's bridges.
From a failure of the flood barrier, to the royal barge's motor stalling or a monumental aquatic pile-up, there are a dizzying number of things that could go wrong.
Organisers have not been able to hold a full dress rehearsal but insist there is only a minimal risk of a crash between the boats, which include several that are more than a hundred years old.
"We want to keep the vessels close enough together to present a spectacle but far enough to avoid the danger of collision," David Phillips, chief harbourmaster at the Port of London Authority, told a London press conference.
"Collision would be the wrong word," added the pageant's director Adrian Evans. "'Kissing vessels' would be more appropriate," he said with a smile.
With the royal family on the river and London swamped by crowds as big as those that turned out for William and Catherine's wedding last year, security will be tight.
Two months ahead of the Olympics, London authorities are under pressure to deliver a smooth event, which apart from the policing has been funded by private donations as the taxpayer struggles under austerity measures.
Some 190 boats will sail alongside the pageant to handle security and respond to any emergencies, while around 5,500 police and 7,000 volunteers will be on standby.
London's Metropolitan Police have planned underwater searches and sniffer dogs will be on the scene, but the force has promised an "unobtrusive" security presence to keep the atmosphere festive.
And if a protester throws themselves into the water, as happened during the University Boat Race between Oxford and Cambridge in April, police say they are "confident" they can manage the incident.
But there is one factor the organisers are powerless to control. Even if proceedings on the river go off without a hitch, the weather could play spoilsport.