Astronauts from the space shuttle Atlantis have successfully completed their third and final spacewalk on the International Space Station.

David Wolf and his British-born colleague Piers Sellers ended a six-and-a-half hour operation at 2047 GMT on Monday to install a new truss on the platform.

You're making it look too easy for us

Mission Control message to astronauts

The truss, or girder, is essential for the ISS to receive giant new power and science modules in years to come.

The six-strong crew of the Atlantis are due to leave the station on Wednesday, returning to Earth on Friday.

They arrived last week with the 14-tonne truss stored in the orbiter's hold.

The $390m aluminium girder contains a new external cooling system for the ISS and is part of a framework that will hold electricity-generating solar wings and radiators needed for future laboratories.

It will form part of a structure that will eventually stretch 107 metres (350 ft) - the longest structure ever built in space - and features:

• A new internal cooling system for the station
• A communications system to enhance voice and data capability
• A cart which will serve as a work platform for future spacewalkers
• Two external television cameras.

There were expressions of delight at the smooth completion of the work, which had been held up on Sunday by an electrical problem.

"You guys are doing a great job," mission control said during Monday's spacewalk.

"Our only concern is that you're making it look too easy for us."

During the operation, the astronauts also managed to retract a bolt which had become stuck in equipment installed earlier.