Doubts over a firm opening date came as design-and-build contractor Mace started full testing yesterday on the £50m project as all 34 cabins of the Emirates Air Line took off for their inaugural load testing flight.

The load tests are part of a series of rigorous safety trials following last month’s completion of the three towers with cabling extended across the river between the O2 and Excel centres.

The cabins travelled across 1.1km of cable, 90m above the Thames with weights representing the maximum number of passengers before the Emirates Air Line opens for passenger service in “summer 2012.”

Tfl officials said they were currently working on the basis that the cable car would not be ready in time for the Games.

Mace is working closely with cable car specialist Doppelmayr to carry out the testing.

Matt Randall, Project Director for Mace, said: “The testing and commissioning of all 34 cabins over the Thames will see the system really put through its paces over the coming days.

“The cabins have travelled smoothly on their early journeys and we are making excellent progress on the wider construction with both Terminals and landscaping nearing completion.”