The fruit of their research is that sales of illegal drugs and sexual services add around 10 billion pounds ($16.7 billion) to Britain's economic activity each year, making up just under 1 percent of total economic output.

Britain's Office for National Statistics published the figures on Thursday alongside a detailed account of its methods, as it prepares to move Britain's public accounts to a new European Union model in September.

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The changes cover far more than black-market activities, and in total are likely to add around 4 to 5 percent to the level of gross domestic product as new businesses are added to the economy and the contribution of old ones reviewed, the ONS said.

Finding accurate estimates of Britons' consumption of drugs and prostitution has proven the trickiest part of the job.

"The estimates are based on data of variable quality, with the estimates of illegal drugs activity markedly stronger than those of prostitution, but both definitely weaker than the estimates of legal activity," the ONS said.

Prostitution is legal in Britain, but brothels, pimps and advertising sex are not, making estimating the number of prostitutes especially complicated.

The ONS said it believed there were at least 58,000 prostitutes in Britain in 2004 - based on a charity's estimate of the number of prostitutes in London - and that numbers since then had increased in line with demand, which it based on the growing number of British men aged over 16.

For other figures, the ONS turned to Dutch research on the number of clients a typical prostitute has per week, as well as how much she or he spent on job-specific clothing and condoms — 125 euros ($170) a year and 50 cents per client respectively.

Prices came from a website where customers review British prostitutes, and they were inflation-adjusted using prices for lap-dancers and escorts already collected by the ONS.

By comparison, working out the volume of crack, heroin, cocaine, amphetamines, ecstasy and cannabis sold was easier, due to existing government estimates of the number of drug users.

Figures for an average drug user's consumption were harder to find, however. Two sources originally used for the price and purity of drugs — a United Nations survey and a government forensics laboratory — were no longer available, the ONS said.

The inclusion of illegal drugs in official data will also widen Britain's trade deficit, as the ONS assumes almost all drugs are imported.

But half of cannabis is assumed to be grown in Britain, which the ONS said would offer a small boost to Britain's farming and pharmaceuticals sectors.

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