China, Turkey and Thailand are among the countries which have tried to pressurise the web company into removing links or videos, Mr Drummond told a London audience.

He acknowledged they did sometimes agree to block access to information in some countries.

"We always do it in as narrow a way as possible... and we only do it locally, not worldwide," he said.

When possible, Google posts details of the requests they receive on a site called Chilling Effects, Mr Drummond added.

He said transparency was vital if Google was to keep the trust of its users.

But the vice president for corporate development warned that attacks on free speech are becoming more common as countries copy the censorship strategies of their neighbours.

He wants governments who support freedom of expression to intervene and put pressure on other leaders when they negotiate treaties.

Google hopes web freedom can become a demand from one country to another when they sign trade agreements.

"In China, censorship is