London Mayor Boris Johnson will today officially open the public viewing deck at The Shard - the highest vantage point in western Europe.
Mr Johnson will cut a ribbon 800ft above London before visitors are welcomed up to take in the view.
He will be joined on level 72 of London's tallest building by its architect Renzo Piano for the opening of the attraction, called The View from The Shard.
On a clear day, visitors to The Shard - which towers over the city at a height of 1,016ft - are promised a spectacular 40-mile panoramic view of the capital.
But forecasts of wet and cloudy weather mean visibility could be severely restricted for the 4,800 people who have already paid for admission today - at £24.95 for adults and £18.95 for children.
According to The View from The Shard website, there will be no refunds.
Instead, visitors will be able to return at another time if the bad weather fails to clear.
But there was better news for those planning a visit tomorrow, which is expected to be a largely sunny day.
Then, viewers should have a clear, 360-degree, 40-mile view of the capital's skyline, from a platform almost twice the height of any other in the city.
With the help of specially designed telescopes they should be able to pick out London's landmarks dotting the streets below.
Visitors to the attraction - on floors 68, 69 and 72 - will be whisked up in two high-speed lifts within around 60 seconds.
The Shard's developer Irvine Sellar and Anders Nyberg, chief executive of The View From The Shard, will join Mr Johnson for the opening.
The mayor and Mr Sellar will make short speeches before Mr Johnson cuts the ribbon at 9am.
The Shard facts:
- The Shard is is 1016ft (309.6m) high
- The building reached its top height on June 19
- It has 11,000 glass panels
- There are 44 lifts, including double-decker lifts
- There are 306 flights of stairs
- The total floor space is 31.4 acres
- It is the 59th tallest building in the world