Three of the UK's 3G networks are going head-to-head in a bit to tempt customers to make use of mobile internet access by slashing prices to £5 per month for a 2.8 megabits per second (Mbps) connection.

The Broadband Lite service, which is currently being offered by 3 is an 18-month contract that features 1GB of downloads per month and the latest Huawei E169G high-speed downlink packet access (HSDPA) modem or 'dongle' thrown in for no extra charge.

It can be difficult to draw direct comparisons between the networks because so many different packages are available, but 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, which has a 3GB data limit, now seems extravagant at £15 per month.

As with all mobile broadband offers, there's a slight catch in the 3 offer: 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 with conventional fixed-broadband services. For mobile users, it's a viable alternative to using Wi-Fi internet connections that can be expensive and difficult to find.

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 1.46Mbps - some way below the speeds claimed by providers. In fact, 3 came bottom of the table for speed, achieving an average throughput of just 1Mbps, while T-Mobile scored 1.1Mbps and Vodafone a still rather modest 2.3Mbps.

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

"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.