Microsoft has underlined the growing concerns about the long-term economic viability of "free" internet portals by indicating that it may soon seek to charge users of its hugely popular MSN site.

Microsoft plans to develop extra services on MSN and then charge users up to £60 per year for using the site. Neil Holloway, managing director of Microsoft UK, said www.MSN.com and www.MSN.co.uk received more than 8.5m unique visits in December, making them Britain's most visited sites ahead of rivals such as Yahoo and Freeserve.

MSN receives 200m visits every month on a worldwide basis. Even so, it is thought to be unable to transform those visits into a profit from advertising and sponsorship revenues alone.

Demanding that users pay to receive the service would make it profitable almost immediately if surfers were prepared to pay. Subscription models for online content sites have not met with much success. The Wall Street Journal and several pornography sites are among the few exceptions.

A decline in online advertising spending and a slowdown in the wider economy has forced leading content sites to look at introducing charges, at least for premium services. Hotmail, the free email service used by tens of millions of people worldwide, is likely to remain free but fees could be charged for more specialised services.