In a strongly worded warning, the US government advised people visiting Beijing for the start of the Olympic Games that they should trust no-one with their electronic devices.

The National Counterintelligence Executive warning was issued on Thursday and warns that while government officials and senior business executives are most at risk, no-one should consider themselves too small a target.

“Security services and criminals can track your movements using your mobile phone or PDA and can turn on the microphone in your device even when you think it’s off. To prevent this, remove the battery,” the warning reads.

“Security services and criminals can also insert malicious software into your device through any connection they control.”

Travellers are advised to not take electronic devices into the country unless absolutely necessary. They should also assume that their hard drive has been copied if the device is examined by customs or in their hotel room if it is searched.

It also warns about the use of USB thumb drives in China, saying they may have malware installed on them. Travellers are also advised to change all passwords immediately on their return home.