THE economies of Belgium, Portugal and Greece are about 20 per cent larger than current figures suggest, according to European Commission statisticians.

Eurostat, the statistical wing of the Commission, is preparing revised, harmonised data to take account of this problem, which occurs partly because tax evasion and other questionable economic activity is not being assessed correctly.

Upward revisions could make it easier for Belgium and Portugal to join in economic and monetary union by meeting the Maastricht criterion for deficits and debts.

However, since Eurostat does not plan to publish the revised data for at least two years, the figures cannot be used for judging which countries qualify for the first wave of a single currency, because that decision will be taken in early 1998.

The revised data is likely to boost several countries' gross domestic products (GDP), leaving the deficit and debt as a proportion of GDP much smaller.

Belgium, for example, would probably meet the Maastricht criterion which stipulates that a deficit be no more than 3 per cent of GDP in 1997, if its overall economy was 20 per cent larger.

Since the EU budget contributions are also calculated according to the size of the economy, the revisions could also mean that Greece, Portugal and Belgium need to pay more to Union funds.

The Eurostat study of the underground economy is one part of a broad attempt to harmonise figures in the region.

Although it does not attempt to measure clearly illegal activity like prostitution, it is engaged in a study to measure informal economic activity, such as market trading.

Italy revised its data about 10 years ago to include an estimate for such an informal sector a change which increased the size of its economy by about 17 per cent.

Officials suspect that if these changes are applied across the continent, the EU economy could be up to 10 per cent larger than currently shown.