The first Tube passengers will arrive at Heathrow Terminal 5 today, on the newly extended Piccadilly line's T5 station.
The new station will also be used by Heathrow Express services, and is the first tube extension to the Tube network since the Jubilee line was extended to Stratford in 1999.
Commissioner for Transport for London Peter Hendy said: "Customers will now benefit from fast, frequent and reliable services to T5.
"Travelling from central London to T5 by public transport is the sensible and practical choice."
Also at the opening, London Underground Managing Director Tim O’Toole,said: "We are transforming the Tube and delivering work when we say we will. It is a mammoth task that will take a number of years but this is further evidence of our progress and the promise of the future."
Tube Lines will have completed its upgrade of the Piccadilly Line by 2014, installing new trains, a new signal control centre and a new line control centre.
Mike Challis, Manager of the Piccadilly line said: "This new and better service will provide direct journeys for passengers between Heathrow T123 and Heathrow T5, bypassing Heathrow T4. Services to T4 will remain the same as they have since we introduced our new timetable in January this year in preparation for this opening."