Europe’s flagship robotic rover mission to Mars now looks certain to leave Earth in 2018, two years later than recently proposed, the BBC understands.
The ExoMars vehicle is intended to search the Red Planet for signs of past or present life.
The delay is the third for the mission originally planned to launch in 2011.
While the switch will disappoint many people, officials say the change will open up a greatly expanded programme of exploration at the Red Planet.
The European Space Agency (Esa) will now join forces at Mars with the US space agency (Nasa). The two organisations believe they can achieve far more by combining their expertise and budgets.
The basis for this approach was agreed at bilateral discussions in Plymouth, UK, last month.
Since then, scientists and engineers on both sides of the Atlantic have been working up the basic architecture for a series of missions in 2016, 2018 and 2020 (launch opportunities to Mars come up roughly every two years).