The veteran singer had to pull out due to a chest infection and bronchitis and Will stepped in to take his place at the last minute.
He said: ''I'm sad that Tom Jones can't do the show, I hope he gets well soon, and am very happy and excited to be playing Hyde Park for the Olympic events tomorrow''.
Will is set to appear on the main stage at the London venue, in between broadcasts on giant screens showing the best of the day's sporting action.
Newton Faulkner is also scheduled to perform tomorrow.
Tom has also had to pull out of two performances at Osborne House, Isle of Wight because of his illness.
The 72-year-old Welshman apologised to his fans on twitter but said it was doctor's orders.
He wrote: ''Sadly I have contracted a chest infection w/bronchitis & due to doctors orders I've had 2 cancel tomorrows and Sundays show. Sorry to all x
''Thanks for all the lovely messages, I don't know if everyones understanding and support is making me feel better or worse! You're the best! (sic)''
BT London Live is a partnership between the Mayor of London, The Royal Parks and The London Borough of Tower Hamlets, and is a part of the citywide programme of events being organised by the Mayor of London to celebrate the London 2012 Games.