Tube services to Heathrow Airport's terminal 4 have resumed after 20 months of work to extend the line.
The Tube line will eventually serve terminal 5
A section of the Piccadilly Line closed in January 2005 while a junction was built to connect the rail link to the new terminal five building.
Passengers can now travel via Tube from Hatton Cross to terminal 4.
Mike Challis, of London Underground, said it was an important milestone in the project to extend the line to terminal 5.
He said: "I'd like to thank... passengers for their patience during the essential 20-month closure.
"The construction work now completed means we are one step closer to a brand new Tube line for a brand new terminal by 2008."
In order to join up the Piccadilly Line with the T5 extension a new rail junction has been built west of terminals 1,2,3, where the new Piccadilly Line tunnels would join the existing Heathrow loop.
During the 20-month closure the terminal 4 station has been upgraded with extra security and a new customer announcement system.