MSL is Nasa's next rover mission to the Red Planet
The US space agency (Nasa) has delayed the launch of its Mars Science Laboratory rover mission.
MSL was scheduled to fly next year, but the mission has been dogged by testing and hardware problems.
The rover's launch would now be postponed until late 2011, agency officials said.
The mission is using innovative technologies to explore whether microbial life could ever have existed on the Red Planet.
The delay could add $400m to the price tag, which is likely to top $2bn.
"Trying for '09 would require us to assume too much risk, more than I think is appropriate for a flagship mission," said Nasa's administrator Michael Griffin.
The launch date was changed following an assessment by the mission's scientists and engineers of the progress it has made in the past three months.
"Despite exhaustive work in multiple shifts by a dedicated team, the progress in recent weeks has not come fast enough on solving technical challenges and pulling hardware together," said Charles Elachi, director of Nasa's Jet Propulsion Laboratory in Pasadena, US.
"The right and smart course now for a successful mission is to launch in 2011."
MSL will use novel technologies to adjust its flight while descending through the Martian atmosphere, and to set the rover on the surface by lowering it on a tether from a hovering platform.
It is engineered to drive longer distances over rougher terrain than previous rovers and contains a science payload 10 times the mass of instruments on Nasa's Spirit and Opportunity Mars rovers.
"Up to this point, efforts have focused on launching next year, both to begin the exciting science and because the delay will increase taxpayers' investment in the mission," said Doug McCuistion, director of Nasa's Mars exploration programme.
"However, we've reached the point where we can not condense the schedule further without compromising vital testing."
Engineers have struggled with the development of MSL's complex actuators - the motors that drive and turn the rover's wheels, and operate its robotic arm.
The window for a 2009 launch ends in late October. The relative positions of Earth and Mars are favourable for flights to the Red Planet only a few weeks every two years.
The next launch opportunity after 2009 is in 2011. The window in 2011 runs through October to December.
Dr Ed Weiler, chief scientist at Nasa, announced he had held discussions with the European Space Agency (Esa) about conducting joint missions to Mars in future. He said the cost of such missions meant collaboration was inevitable.
Dr Weiler told reporters that preliminary discussions with his opposite number at Esa, David Southwood, had led to an informal agreement that in future they would adopt a joint architecture for all missions to the Red Planet.
Europe and the US could now make ExoMars a joint mission
Both agencies are likely to combine their efforts in the early 2020s to return rocks from Mars for study in Earth laboratories.
Europe has already made a decision to delay the launch of its own Mars rover, ExoMars, from 2013 to 2016.
Dr Weiler said there was a possibility this mission could also now become a joint venture with Nasa even though ExoMars is quite advanced in its design.
"We had a very short discussion yesterday on some ideas on how we could work together on ExoMars. They are literally at the viewgraph stage at this point in time, and I think we both learned not to make too many plans based on Powerpoints," said Dr Weiler.
"It is going to take some real scientists and engineers getting together and working that out. But is there a possibility it could become a joint mission? Absolutely. And we're certainly open to it and would welcome it."
At its Ministerial Council meeting last week, Esa said it was actively seeking the participation of both the US and Russia on ExoMars as a means of limiting the mission's 1.2bn-euro cost.
David Southwood told the BBC that international cooperation at Mars was the only way forward.
"This is big," he said. "Ed and I can set the grand plan, but we need our people to get together to work out the detail. Give us six months and we'll have an announcement."