Gamblers and buyers of pornography were yesterday causing a headache for statisticians striving to find out how much Britons spend via the internet.

National Statistics (ONS) said its data captured most spending by households and business online - but it needed to do further work in areas such as gaming, music down-loads and the purchase of adult material.

Sales of goods and services to households over the internet by firms outside the financial sector reached £18.1 billion in 2004 - up 67 per cent on the previous year.

According to the ONS, this figure accounted for about 2.5 per cent of the total spending by households.

Although most internet spending was captured in the National Accounts, the ONS said: "There are some areas where future work is needed to ensure coverage is as good as possible."

The most important area was the increasing use of the internet to buy goods from abroad, which the ONS said could affect traditional trade figures.

The ONS said: "Other areas include looking more closely at how downloads of ringtones and music are captured. These markets are still quite small but are beginning to grow rapidly.

"Gaming subscriptions, online gambling, software downloads and purchasing adult material are other areas that will need to be

considered. All of these are complicated by the global nature of such online activity and purchasing."

The boom in online gambling was catapulted into the headlines last year by the £4.64 billion stock market flotation of poker firm PartyGaming along with smaller rivals 888 Holdings and Empire Online.

888 Holdings - the operator of the world's largest online casino - said at the time of its September listing that the internet gaming market was expected to turn over $12 billion (£6.7 billion) in 2005 and double this amount by 2010.

According to Global Betting & Gaming Consultants, online gaming currently accounts for six per cent of all gambling revenues in the UK.

Investec economist David Page said the overall tone of the statement from the ONS was "comforting" because it signalled that statisticians were confident that UK consumption was being recorded accurately.

Most goods purchased online from retailers are captured by the retail sales index - a monthly survey of 5,000 businesses that include all the major chains and some smaller shops.

Citing Tesco as an example, the ONS said this survey would collect the total sales of the super-market giant as a whole without breaking out internet spending separately.

Tesco saw internet sales in the UK hit £719 million in 2004/5.