The following correction was printed in the Guardian's Corrections and clarifications column, Saturday January 28 2006

In the article below we described Falun Gong as a cult. In doing so, we should have made clear that we were giving the Chinese government's official view of the movement.

Google, the world's biggest search engine, will team up with the world's biggest censor, China, today with a service that it hopes will make it more attractive to the country's 110 million online users.

After holding out longer than any other major internet company, Google will effectively become another brick in the great firewall of China when it starts filtering out information that it believes the government will not approve of.

Despite a year of soul-searching, the American company will join Microsoft and Yahoo! in helping the communist government block access to websites containing politically sensitive content, such as references to the Tiananmen Square massacre and criticism of the politburo.

Executives have grudgingly accepted that this is the ethical price they have to pay to base servers in mainland China, which will improve the speed - and attractiveness - of their service in a country where they face strong competition from the leading mandarin search engine, Baidu.

But Google faces a backlash from free speech advocates, internet activists and politicians, some of whom are already asking how the company's policy in China accords with its mission statement: to make all possible information available to everyone who has a computer or mobile phone.

The new interface - google.cn - started at midnight last night and will be slowly phased in over the coming months. Although users will have the option of continuing to search via the original US-based google.com website, it is expected that the vast majority of Chinese search enquiries will go through mainland-based servers.

This will require the company to abide by the rules of the world's most restricted internet environment. China is thought to have 30,000 online police monitoring blogs, chatrooms and news portals. The propaganda department is thought to employ even more people, a small but increasing number of whom are paid to anonymously post pro-government comments online. Sophisticated filters have been developed to block or limit access to "unhealthy information", which includes human rights websites, such as Amnesty, foreign news outlets, such as the BBC, as well as pornography. Of the 64 internet dissidents in prison worldwide, 54 are from China.

Google has remained outside this system until now. But its search results are still filtered and delayed by the giant banks of government servers, known as the great firewall of China. Type "Falun Gong" in the search engine from a Beijing computer and the only results that can be accessed are official condemnations.

Now, however, Google will actively assist the government to limit content. There are technical precedents. In Germany, Google follows government orders by restricting references to sites that deny the Holocaust. In France, it obeys local rules prohibiting sites that stir up racial hatred. And in the US, it assists the authorities' crackdown on copyright infringements.

The scale of censorship in China is likely to dwarf anything the company has done before. According to one internet media insider, the main taboos are the three Ts: Tibet, Taiwan and the Tiananmen massacre, and the two Cs: cults such as Falun Gong and criticism of the Communist party. But this list is frequently updated.

In a statement, Google said it had little choice: "To date, our search service has been offered exclusively from outside China, resulting in latency and access issues that have been unsatisfying to our Chinese users and, therefore, unacceptable to Google. With google.cn, Chinese users will ultimately receive a search service that is fast, always accessible, and helps them find information both in China and from around the world."

It acknowledged that this ran contrary to its corporate ethics, but said a greater good was served by providing information in China. "In order to operate from China, we have removed some content from the search results available on google.cn, in response to local law, regulation or policy. While removing search results is inconsistent with Google's mission, providing no information (or a heavily degraded user experience that amounts to no information) is more inconsistent with our mission."

Initially, Google will not use Chinese servers for two of its most popular services: Gmail and blogger. This is a reflection of the company's discomfort with the harsh media environment - and the subsequent risks to its corporate image.

In an attempt to be more transparent than its rivals, Google said it would inform users that certain web pages had been removed from the list of results on the orders of the government. But its motivation is economic: a chunk of the fast-growing Chinese search market, estimated to be worth $151m (£84m) in 2004. This is still small by US standards, but with the number of web users increasing at the rate of more than 20 million a year, the online population is on course to overtake the US within the next decade.

Julian Pain of Reporters Without Borders - a freedom of expression advocacy group that also has its website blocked in China - accused Google of hypocrisy. "This is very bad news for the internet in China. Google were the only ones who held out. So the Chinese government had to block information themselves. But now Google will do it for them," he said. "They have two standards. One for the US, where they resist government demands for personal information, and one for China, where they are helping the authorities block thousands of websites."

Local bloggers were already wearily resigned to the change. "What Google are doing is targeting commercial interests and skirting political issues," said one of the country's most prominent, who writes under the name Black Hearted Killer. "That by itself is no cause for criticism, but there is no doubt they are cowards."

Forbidden searches

Words or phrases that can trigger pages to be blocked or removed from search results:

Tiananmen Square massacre

The killing of hundreds, if not thousands, of civilians by the People's Liberation Army in 1989

Dalai Lama

The exiled spiritual leader of Tibet, who is denounced as a splittist by the government in Beijing

Taiwanese independence

The nightmare of the Communist party, which has vowed to use force to prevent a breakaway

Falun Gong

A banned spiritual movement, thousands of whose members have been imprisoned and in many cases tortured

Dongzhou

The village where paramilitary police shot and killed at least three protesters last month