There are any number of problems that can cause a website to struggle in times of exceptionally high demand. Most, however, can be prepared for, and experts said there was no excuse for the HRMC site not to have been able to cope with what it knew would be its busiest day of the year.

One is bandwidth - the size of the pipe that connects a website with the internet. Typically there is a peak amount of information - measured in megabits per second (Mps) - that can be communicated to a site's visitors as they request pages at any one time.

Using a technique called 'bursting', however, the owner of the site can call upon its service provider to increase its bandwidth spontaneously as demand changes. Around sale time, for instance, a retail website may increase its bandwidth from the usual 10Mps to 100Mps.

Another is the site's 'web servers' - the boxes which store all the information on the site and fetch pages when a visitor requests them. Each box - typically an inch high, with the dimensions of a DVD player - has a limited amount of requests it can handle at once.

A site aware that demand is about to increase can either rent more servers in advance or, as increasingly happens, rely on a hosting company like IBM to run their servers, in which case the number operating at any one time can be increased automatically. This technique is known as 'on-demand' computing.

The problem may also rest with something called the site's app servers. These are like web servers, but perform more complicated tasks.

For instance, in HRMC's case, there may be features on the site which allow it pull information out of the HRMC database relevant to the visitor. If the software running on these servers is not able to 'scale' to meet the demands of the visitors, then the performance of the site can be affected.

One technique developed in recent years is 'content distribution' - where sites with global audiences store information on servers around the world, allowing local users to access material nearer them, thereby easing pressure on the main server.

On the day of the July 7 bombings, for instance, the number of visits to the BBC's website rose from 35 million to 115 million, but the site was able to cope by relying on third party servers.

"Sometimes a company can throw all the kit in the world at their site in an attempt to fix it, but if they don't know where the problem is, it won't make any difference," Gareth Evans, client services manager with the quality assessment firm SiteMorse, said.

Sites can now work out whether they are prepared to cope with periods of high demand using a technique called 'load testing', where software is used to simulate a high number of visits.

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