The incident occurred last week after the checks began but "the problem disappeared spontaneously," ESA said in a status report.

Engineers have decided to postpone the remaining tests until the first week of July to give themselves time to figure out what went wrong, it said.

On Monday, the Russian space agency, Rosaviakosmos, said that the Mars mission had been hit by a major problem.

It said there had been a breakdown in communications between the orbiter, Mars Express, and Beagle-2, a British-built lander which it carries.

But ESA denied this and said everthing was now working properly.

"These kind of events are considered routine in a space mission, but engineers would like to understand the causes before restarting the instrument tests," it said.

Mars Express was launched on June 2, heading a trio of European and US robot missions to the Red Planet that will, if things go well, be joined by a Japanese probe at the end of the year.

Beagle-2 is a small stationary lander carrying instruments that seek to confirm suspicions, drawn from pictures by orbiting American probes, about the presence of water on Mars.

At present, the lander is attached to the belly of Mars Express. They are scheduled to separate when they get close to their destination in December, and Beagle-2 is likely to land on December 25.

The instrument checks are a routine operation to make sure that all the equipment has survived the rigours of liftoff.

The "temporary anomaly" occurred in a unit called the solid state mass memory (SSMM), which stores data from the instruments prior to sending them to Earth, ESA said.

During a test of one of the Mars Express instruments, the SSMM started to send erroneous data, only for the problem to disappear, it said.

SPACE.WIRE