Charles Bolden, Nasa's head, has been pushing the case for the US to regain the ability to send crew to the station from American soil.

Last year he warned congress that continuing spending cuts could leave America dependent on the Russians.

It had been hoped that the US could develop a domestic capability by next year, but Congress halved the budget put forward for the programme by the Obama administration.

Last week he appeared to have won his battle when the White House announced the extension of the Space Station's life until 2024.

"Launching American astronauts to the space station from US soil has also been a top priority of the Obama Administration, and we're making great strides toward certifying private companies to transport our astronauts into orbit," the White House said.

"With the first commercial crew flight scheduled for 2017, some had questioned the value of a commercial crew investment that would have lasted only three years.

"Extending ISS to 2024, with a concomitant increased number of flights, will drive down the per-flight cost and make this investment even more attractive."

Nasa is in talks with four companies over the crew contract: Boeing, Sierra Nevada, Blue Origin run by Jeff Bezos of Amazon and SpaceX.

The companies have until May to complete the initial designs for the taxis which must be capable of reaching an altitude of 230 miles and remaining in orbit for three days.

"Nasa is looking for a contract with a company like SpaceX,'' explained Jonathan McDowell, a scientist at the Harvard-Smithsonian Center for Astrophysics.

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Nasa looking for 'taxi firm' to replace Russian rockets