Internet users in a number of big Chinese cities have been suffering serious difficulties accessing Microsoft's Hotmail e-mail service, just three weeks after Hu Jintao, Chinese president, was fêted at Bill Gates's Seattle home.

The persistent problems over the past week have led many Chinese Hotmail users to assume the service has been added to the long list of overseas websites blocked by the government.

Beijing never comments on whether it is blocking sites and it can be difficult to distinguish government disruption from network problems in China's creaky internet infrastructure.

The problems are a setback for Microsoft, which is keen to build its online presence in China. If they are the result of Chinese government action they would put Mr Gates in an awkward position as Mr Hu dined at his home during his recent visit to the US.

While there, Mr Hu declared: "Bill Gates is a friend of China and I am a friend of Microsoft." In response, Mr Gates jokingly said he was ready to help the Chinese president if he needed advice on using Microsoft's Windows software.

Subscribers to foreign e-mail services in China often report problems accessing accounts but Hotmail users in Shanghai, the central city of Chongqing, Beijing and north-eastern Shenyang said have had acute difficulties recently. Several users reported that the situation had improved by yesterday but many contributors to Chinese online discussions about the access problems were in no doubt that government action was to blame for the problems.

A Chinese official of Microsoft's online MSN service acknowledged that many users had complained about being unable to access its messenger service and Hotmail.

Engineers were investigating the disruption to the service, the official said.

MSN launched a locally based business in China last year and drew criticism for banning words such as "democracy" and "freedom" from parts of its website in an apparent effort to avoid offending Beijing.