Third-generation (3G) networks are rapidly becoming one of the cheapest ways to get broadband of any kind in the UK with the 3 network cutting the price of a 2.8Mbits/s connection, to only £5 per month for existing customers.

The 3 network’s £5 Broadband Lite service includes 1GB of downloads, an 18-month contract length, and the latest Huawei E169G HSDPA modem – sometimes confusingly called a “dongle” – thrown in for free.

Direct comparisons are difficult to make because of the welter of options, but rivals Vodafone and Orange charge £15 a month for a service based on 3GB of downloads per month, while O2 charges £20 per month for roughly the same. Contract periods for decent tariffs are usually at least 18 months, with charges for HSDPA mobile modems and usable throughput speeds varying depending on network.

Even T-Mobile’s once innovative Web ‘n’ Walk Plus service now looks dear at £15 per month, though that has a 3GB data limit.

As with all mobile broadband offers, the slight catch in the 3 offer is that to take advantage of the price, you have to be an existing customer of the company’s mobile phone service. The cheapest of these is currently £12 per month with the handset costing extra on top.

Excess downloads cost 10 pence per megabyte over the 1GB allowance. Alternatively, a 3GB and 7GB download limit will set existing users back £7.50 and £12.50 per month, respectively.

New customers can still take advantage of a £10 per month offer for the same service, with pay-as-you-go users now paying only £50 for the Huawei E220 or ZTE MF622 modems, down from £69.99.

Despite the small print, the 3 offer is still impressive – mobile broadband is on paper now competitive when compared to conventional fixed broadband services. For mobile users, it is a viable alternative to using sometimes difficult-to-find and expensive Wi-Fi Internet connections.

It’s not all good news for mobile broadband, or for 3. Last week broadband comparison website Broadband Expert claimed that the average mobile broadband throughput was only 1.46Mbits/s, some way below the speeds claimed by providers. In fact, 3 came bottom of the table for speed, achieving an average throughput of 1Mbits/s, while T-Mobile reached 1.1Mbits/s, and Vodafone a still rather modest 2.3Mbits/s.

Fixed-line networks in the UK routinely claim throughputs of 8Mbits/s, despite plenty of evidence that many users experience something closer to 4-6Mbits/s.

“It is not surprising that mobile broadband is behind fixed line broadband as it’s a comparatively new technology. But, mobile broadband providers must not follow the lead of home broadband services and advertise speeds that are largely unachievable,” said a Broadband Expert spokesman.