Microsoft may begin training the police in ways to break the encryption built into its forthcoming Vista operating system.

The news was revealed in a parliamentary committee session in which Professor Ross Anderson of Cambridge University warned MPs that if such a move was being considered then the police should start learning sooner rather than later.

The need to decrypt hard drives was a prominent reason given for extending the length of time that the police could hold terrorism suspects.

"It is our goal to give PC users the control and confidence they need so they can continue to get the most out of their PCs," said a Microsoft spokeswoman.

"At the same time, we are working with law enforcement to help them understand its security features and will continue to partner with governments, law enforcement and industry to help make the internet a safer place to learn and communicate."

This is not the first time such links between Microsoft and the security services have received attention. In 1999 journalist Duncan Campbell alleged that Microsoft had reached a secret deal with the National Security Agency in the US to allow backdoor access to Microsoft systems.