BROADBAND firms are having to restrict customers’ usage because of the unprecedented success of the BBC’s iPlayer, the online viewing service.

The news will raise fears that Britain’s broadband network is struggling to cope with the growing demand for TV programmes that can be viewed online after they have aired.

Thousands of broadband users face breaching their usage limits as a result and will have to fork out more for superior packages.

The iPlayer – which was launched in December and allows you to watch your favourite programmes on your computer – has attracted 17m people in its first three months. However, it has increased internet traffic by 66%, say some broadband providers.

They have a limited “bandwidth” so, with more people using high-speed services, are having to impose restrictions on speeds, and use download limits and “fair usage policies” to control traffic.

iPlayer programmes tend to be around 300 megabytes (MB) in size though longer one-hour shows like David Attenborough’s Life in Cold Blood can take up 600MB.

However, many low-end broadband packages have monthly download limits of one gigabyte (GB) – equal to about 1,000MB. Downloading just two iPlayer shows a month could therefore mean you use up your limit and have to pay extra for additional downloads.

One Sunday Times reader, Brian Gordon, 60, from Lydney in Gloucestershire, uses the iPlayer to watch history programmes. “I find it much easier to choose when I watch programmes, rather then worry about recording them when I’m out,” he said.

He is with Plus Net, paying £9.99 a month, but soon breached his 1GB limit. He now pays an additional £1.50 a month for 3GB.

Since the launch of the iPlayer, the number of customers using more than 1GB a month has doubled, said Plus Net. It charges 75p a month for each additional GB.

The problem is set to get worse as broadband providers “unbundle” the old BT exchanges by placing their own equipment in them, which will allow them to offer faster speeds and more downloads.

This has raised fears that firms will also take the opportunity to “manage” customers’ use more.

The regulator Ofcom said: “Unbundling does not automatically give you a better service. It does, however, allow for telecoms firms to manage traffic on their networks more effectively.”

Tiscali Broadband, Britain’s fourth-largest provider, has been accused of imposing “traffic management tools” on Pipex customers who are being transferred to the Tiscali network. Tiscali, which bought Pipex last year for £210m, said it would not change the service received by customers. Internet forums, though, suggest Pipex customers face restrictions.

Tiscali customers already experience traffic management during the peak hours of 7pm-11pm and at weekends.

A spokesman said: “We are not altering the Pipex product. We never reduce the speed below 10% of the network capacity. By using traffic management we prioritise things like e-mails.”

The management policy could, however, reduce the speed of a service from up to 8Mb to around 0.8Mb and restrict the use of some services like the iPlayer, Channel 4’s on-demand service and Sky’s Anytime download service.

Some providers offering unlimited downloads also impose “fair usage policies”. Virgin Media’s top-end package costs £37 a month and advertises “unlimited” downloads, but customers could have their speeds restricted if they use more than 3GB between 4pm and midnight.

The package is one of the fastest mainstream services in Britain and allows for a theoretical 20MB download speed, but a typical user could exceed this in 20 minutes, according to comparison site Broadband Choices.

Programmes on the iPlayer can be downloaded to your computer, so you can view them for a month, or “streamed”, whereby you watch as you download. Streaming uses a little less of your download limit as the quality is reduced. Plus Net said a 30-minute show will use around 125MB if you stream – about 40% of the download rate.

It is becoming increasingly easy to use up a supplier’s fair usage limits as more video content becomes available.

An average one-hour session on Facebook uses around 80MB, according to Plus Net. MySpace uses more because it has more video, audio and streaming options so you could be looking at about 400Mb.

One album from iTunes would use about 100MB of usage but Apple’s high definition movie trailers use much more: about 2GB an hour.

The iPlayer is a peer-to-peer service, where data is shared, so other viewers can use up your monthly limit by “uploading” programmes from your PC.