The danger of promoting and launching web sites before testing them properly was illustrated this week when two major projects ran into difficulties. Experts noted that such problems can cause firms to lose custom and visitors.

On Monday Coca-Cola was due to launch its music download site, Mycokemusic.com. However, the site did not go live until Wednesday morning.

Another site due to launch on Monday, Evidenceincamera.co.uk, suffered a similar fate. The site, which features over five million detailed aerial photographs from World War II, was unavailable despite being promoted in the media over the previous weekend.

Sue Pratt, head of e-commerce for IT services and management consultancy Salmon, said comp- anies should not launch sites before they are ready. "Performance testing is absolutely critical," she argued. "With increased numbers of people using the internet, sites have to meet their goals."

David Hill, vice president for Europe, the Middle East and Africa at Spirent Communications, added, "It is absolutely critical for sites to stay up. You have to keep your users happy."

Hill said that Spirent had found companies are increasingly worried about their sites becoming unavailable whether due to under-capacity or as a result of denial of service attacks. But he added that it was relatively simple to assess a site's performance under potential loads.

Hill said that even when traffic exceeded the capacity of web sites, those sites should still be able to serve some users, but in practice this is rarely the case.

Last November firms were advised to learn from the mistakes of the government following the launch of its 1901 census web site, which was crippled by a high volume of traffic. The Public Record Office put the 1901 census online on 2 January 2002, but it grossly underestimated demand. It had expected 1.2 million visitors each day, but it saw that many each hour. After five days the site was taken down.

The web site was not fully online until 11 months after its launch and government watchdog the National Audit Office (NAO) issued guidelines for companies after investigating the project. "It is important that the wider public sector learns the lessons from this project," said Sir John Bourne, head of the NAO in a statement.

The NAO said firms should be careful to accurately predict the likely traffic, and should include a safety margin in case traffic exceeds forecasts. It also recommended that firms look for independent guidance and testing before they go live.