BEIJING (AFP) - China's economy would be at least 3.7 trillion yuan (S$753 billion) bigger than Beijing thinks if the country's local government statistics were to be believed, state media reported on Wednesday.

The Economic Information Daily tallied up gross domestic product (GDP) data from 28 of mainland China's 31 provincial-level authorities, totalling 42.4 trillion yuan for the first nine months of the year.

But the figure for the whole country, already announced by Beijing, is 3.7 trillion yuan lower.

The discrepancy - which has been in place for more than two decades - has been widening rapidly in recent years, the Economic Information Daily said.

The reliability of Chinese economic data has long been in doubt as local officials tend to massage the figures upwards in pursuit of promotion and the newspaper, which is run by the official Xinhua news agency, pointed to the same problem.

"Some regions may have inflated the statistics due to their distorted perception of achievements given the fact that the performance assessment of local governments is often linked with GDP growth," the report quoted an unnamed National Bureau of Statistics official as saying.

China's Premier Li Keqiang said in 2007, when he was the governor of Liaoning province, that some Chinese data was "man-made", according to a confidential memo released by the WikiLeaks website in 2010.

He told US diplomats that he focused on only three figures - electricity consumption, rail cargo volume, and the amount of loans issued - to evaluate his region's economy, the leaked document showed.

Chinese President Xi Jinping said in June that officials' performance evaluations must not be based "simply on GDP growth rate" but take into account factors such as the environment and improving people's well-being.