On the mainland it is not just government that censors negative reports or promotes positive online postings. Mr Unknown, a partner in China News Communication Net, says the erasure of online posts became an industry on the mainland around 2005 and blossomed two years later as the number of internet users boomed.

The mainland had 384 million internet users in December, almost double the number at the end of 2007.

Mr Unknown entered the industry in 2008 and his firm's online introduction boasts of connections with many mainland websites, including portals, official websites and forums, from which it can erase digital information.

He said that to avoid being tracked by police or targeted by competitors, erasers generally contacted clients only via QQ, the mainland's most popular instant messenger service.

Companies, some of them state-owned giants or even multinationals, find erasers through online searches or personal introductions and then pay to have negative posts deleted.

While some erasers claim to have 'special techniques', Mr Unknown said they really relied on guanxi (connections). 'Sometimes the website editors or forum administrators even allow us to sign into their operating platforms,' he said.

But he said it was getting harder for small firms to compete as websites and forums tightened up on deletion policies, and they occasionally had to ask for help from bigger companies, with even better connections.

Some industry insiders said officials from the mainland's internet surveillance authorities might be involved in the business.

'If the officials say that posts about bad service or poor products are a threat to social stability, all websites must delete them,' a senior editor with one of the mainland's biggest portals said.

He said many negative posts about Sanlu, a state-owned dairy firm in Hebei investigated for producing melamine-tainted milk in 2008, had suddenly disappeared from search engines and most websites. The case is politically sensitive because many parents are fighting for compensation for children poisoned by the adulterated milk.

Professor Li Yanhong , from Sun Yat-sen University's school of communication and design, said the booming erasure industry reflected the lack of professional ethnics in the mainland's PR and internet industries.

She said although the internet was not considered part of the media on the mainland, it did communicate information and its employees should adopt rules followed by the press that sought to prevent undue influence from commercial interests.

A senior manager at one of the mainland's biggest car websites said erasers were either people who had friends at websites or were insiders themselves. She said erasers shared connections because online editors changed frequently, making it hard to know them all.

A representative of a Beijing-based online PR company with the QQ identity Feixiang said it would cost 4,000 yuan (HK$ 4,546) to permanently block a link to a negative report on Sohu.com's IT channel, while blocking the same article on Tianya.com's blog platform would cost 2,200 yuan.

Feixiang said it could also push links to negative reports further down the list returned by search engines, but that would cost more. To avoid such a link appearing on the first three pages of a search on Baidu, the mainland's biggest search engine, for six months would cost up to 100,000 yuan.

'You need to pay 50 per cent up front, but we can offer you discount,' Feixiang said.

An employee with Sohu.com's media relationship department said he could not confirm whether the erasers' claims were true. But he said Sohu prohibited such behaviour.

Baidu.com said last week that it had never authorised anyone to delete content from its search page and it was illegal for people to claim they could provide such service.

Mr Unknown said working outside the law allowed erasers to charge high prices. And he said bigger companies were charged more.

'They understand that compared with the profits they might lose as a result of negative reports, the money they pay us is quite small,' he said.

He declined to name any of his clients but according to the firm's online introduction they included a mainland dairy giant and two household appliance manufacturers, one from the mainland and the other from South Korea.

Competition between clients can be a headache. Mr Unknown said his firm once deleted a negative report for a client, only to be criticised by one of the client's competitors, who had posted the negative report.

He said it was time to quit.

'We know it's illegal so it's risky to run the business,' he said. 'And there are too many competitors now, with more pouring in. We have to quit.'

However, others still see online erasure as a lucrative business opportunity. The owner of a Guangzhou-based advertising company said he would like to establish connections at websites and start a similar service.

'A lot of people are doing this,' he said. 'I can see the demand from my current clients, and it is profitable. Why not try?'