European Union countries will be including prostitution and illegal drug sales in official gross domestic product (GDP) figures as from next year.

The move has been driven by new European Union rules requiring nations to include all activities that produce income in their national accounts, regardless of their legality.

The procedure is expected to be "very difficult for the obvious reason that these illegal activities are not reported".

For example, the Bank of Italy in 2012 estimated the value of the criminal economy at 10.9 per cent of GDP.

Theoretically, that could mean Italy's GDP result with the new calculation will come in far higher than the government's 1.3 per cent growth estimate.

Eurostat earlier estimated the average GDP increase for EU nations due to the new calculation to be at 2.4 per cent.

The highest rises were estimated for Finland and Sweden at 4 to 5 per cent, followed by Austria, Britain and the Netherlands at 3 to 4 per cent.

The increase for Italy would be around 1 to 2 per cent.