The six-wheeled robots are claimed to be the most robust and manoeuvrable planetary rovers to be built.

According to Chris Draper, Astrium's ExoMars rover vehicle industrial manager, they can go literally where no rover has gone before.

"Obviously, the American MER rovers (Spirit and Opportunity) that were put up by Nasa enjoyed an extreme amount of success. They were able to travel large distances, well beyond their planned lifetimes. But we're hoping that with our baby, we'll be able to go places that are actually much further."

EXOMARS MISSION CONCEPT Set to leave Earth in 2013; primary aim is to search for life Will launch on a heavy-lift rocket, such as the Ariane 5 Vented landing bags allow for a larger payload Rover will carry a 16.5kg 'Pasteur' instrument suite 30kg geophysics/environment static station also possible This would study the weather and listen for 'Marsquakes' Concept to cost Esa states more than first estimates

Each prototype has six-wheel steering, which means that they can turn all six wheels and crab sideways.

They also have what engineers describe as "wheel walking capability". This means that when the vehicles come across a particularly steep or slippery slope, they can anchor themselves with five of the wheels and inch each wheel forward one at a time, to crawl over an obstacle.

The prototypes have a unique intelligent navigation system which enables them to plot their own course.

Such is the distance between the planets, it can take 20 minutes for an instruction sent from Earth to arrive at Mars. The delay means instant commands to change direction are not possible and so a rover must have autonomy to make decisions if, for example, it is approaching a dangerous precipice.

And because the rover will make its own decisions, it can also cover more ground.

ExoMars' primary mission is to search for signs of past or present life.

To do that, it will make its way to locations thought most likely to support life and drill up to two metres into the ground. Sub-surface soil samples will be analysed by an onboard laboratory.

The rover will have the largest array of scientific instruments to be taken to Mars. So if it gets strong indications that organisms might be present, it will be able to subject samples to a wide range of tests to confirm initial readings.

But the executives at Astrium are concerned that after having done the groundwork in developing the vehicle systems (the chassis, locomotion and navigation systems), the company might actually lose the opportunity to build the final flight vehicle.

It's quite a tense period for us in Astrium and for the other companies involved in building ExoMars Dr Ralph Cordey, UK Astrium

The concern stems from the recent funding difficulties faced by the Science and Technologies Facilities Council (STFC), the agency which supports astronomy and space science in the UK.

The company won its role on the mission as leader of vehicle development partly because of the funding the STFC put through Esa. But Astrium's science business development manager, Dr Ralph Cordey, said that given the recent squeeze on British astronomy and space resources, he was now unsure if that same level of funding would continue.

"It's quite a tense period for us in Astrium and for the other companies involved in building ExoMars; and for all the many, many scientists who will depend on this mission," he told BBC News.

"On the one hand, we are moving ahead with a great momentum, as you've seen. This is not a paper project - there is real engineering going on and we are on the verge of building the real hardware that will go on to the planet Mars.

"But on the other hand, we do have a real problem. In order to complete the project, further funding needs to be made available and it won't be until November that we are sure that that is in place."

The cost of ExoMars has risen dramatically since European space ministers first approved the venture in 2005. Revised estimates to be agreed next month with industrial partners mean the price for ExoMars will nearly double from its original 650m-euro price tag.

If the UK wants to maintain its position on mission, it will have to boost its financial commitment to ExoMars or come to some other compensatory arrangement with Esa.

If the UK plays the wrong hand at the next space ministers' meeting in November, UK Astrium warns, the contract to build the flight vehicle could go to a space company sited in another Esa member state.

The STFC said: "The UK will be working with its international partners to try to bring about a mission that has high impact and is affordable, in a process of negotiation that will culminate in decisions at the ministerial in November 2008.

"Meanwhile, it remains a high strategic priority for the UK as emphasised previously."