After exhaustive testing Finnish security firm F-Secure has failed to make a virus leap from a mobile phone handset to a car's onboard communications system.

F-Secure did the tests in response to rumours that some Lexus cars had been infected by a virus.

But the phone system on the vehicle did not respond to any of the attacks tried out by F-Secure researchers.

Test bed

Many security firms fear that the increasing number of computers and communications system on cars will eventually make them vulnerable to the viruses that plague desktop machines.

In January this year stories began circulating that some models of Lexus Landcruiser, particularly the LX470 and LS430, were vulnerable to phone viruses that travel via the Bluetooth short-range radio system.

The vehicles use Bluetooth so owners can use their phone-based address book with the in-car phone.

Toyota issued a statement saying that its in-car communications system could not be infected because it did not use the Symbian operating system - which the mobile phone viruses exploit.

However, Finnish security firm F-Secure decided to put the claims to the test and subjected a Toyota car to a barrage of phone virus attacks.

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The attempts to infect the car were carried out in an underground testing chamber to guard against the chance of accidental infection.

The F-Secure researchers used phones compromised with the Cabir virus to see if they would infect the car too. All the attempts at infection failed.

"No matter what we did the car did not react to the Bluetooth traffic at all," wrote F-Secure researcher Jarno Niemela.

The car did not even react when F-Secure used a special program to transfer the Cabir file.

The only problems with the car emerged when the battery was drained by all the testing. The only problem that F-Secure found was caused by a corrupted phone name.

But generally, said the researchers the Prius stood up to the attacks very well.

"We had to reboot our test systems several times as their Bluetooth systems died on us, while Toyota Prius just kept going," said F-Secure.