American internet companies have been criticised by politicians and advocates of free speech for acceding to central government demands that they filter content Beijing regards as politically sensitive, such as references to Tibet, Taiwan and the 1989 Tiananmen Square crackdown.

'We at Google have a mission to serve all of the citizens of the world, and that mission includes the more than 1 billion people here in China,' Mr Schmidt replied to repeated questions on the company's attitude to censorship.

Google founders Larry Page and Sergey Brin pledged in the company's charter to 'do no evil', a promise that has exposed them to more criticism than their competitors, such as Yahoo and Microsoft, which also comply with strict mainland censorship rules.

'It's not an option for us to broadly make information available that's illegal or inappropriate or immoral. We simply don't have a choice,' Mr Schmidt said.

Websites blocked on the mainland include Wikipedia and the BBC.

Some defenders of Google have said the company does more for the cause of free speech in China than its competitors by including a short message at the bottom of sensitive search results informing users that some results cannot be displayed because of local laws, rules and policies.

The company has focused its attention on China as the fastest growing internet market in the world and has said it must conform to Beijing's demands if it wants to operate there. However, critics say Google and other US internet providers are putting the quest for profits ahead of ethics