Disaster droids face off in the Darpa Robotics Challenge

Final preparations are being made by 24 robotics teams about to compete for $3.5m (£2.3m) worth of prizes in a Pentagon-backed competition.

Their machines are being challenged to carry out a series of disaster-related tasks in the shortest time possible.

The US Department of Defence's Darpa research unit will host the event on Friday and Saturday at a location close to Los Angeles, California.

A team from Google was a favourite to win, but has quit the contest.

The squad of engineers' Schaft robot won the contest's previous round in 2013, not long after their start-up was acquired by the search firm.

But the BBC understands that Google had concerns about being linked to a military-backed event, even though it is has a "humanitarian" theme.

Google is, however, allowing six other teams to use a new version of Atlas - a robot made by its Boston Dynamics division. They will each power their copy with their own software.