Their stay on the International Space Station (ISS) has taken them past the previous record of 188 days and four hours put down by Shannon Lucid on the old Mir platform in 1996.

Bursch and Walz were asleep at the moment they made history.

The two astronauts are due to return to Earth on the shuttle Endeavour on Monday.

Their mark is still well short of the world record set by Sergei Avdeyev. The Mir veteran spent a total of 747 days, 14 hours and 22 minutes aloft on three separate missions.

On his last trip to Mir, the Russian stayed 379 days in orbit.

Future success

Two other astronauts, who came up with the orbiter, have completed their second spacewalk to continue construction work on the ISS.

Franklin Chang-Diaz and Philippe Perrin wired up and bolted down a work platform that will allow the space station's 17.4-metre (58-foot) robot arm to roam across the exterior of the outpost.

This manoeuvrability is essential to future construction work, when massive modules brought up by shuttles will need to be grasped and moved across the station to be fitted to their permanent positions.

Tuesday's spacewalk was the second of three planned during Endeavour's visit. Chang-Diaz and Perrin will go out again on Thursday to replace a seized wrist-joint in the robot arm.