Hundreds of employees at China's top statistics bureau are being investigated after selling data, the government said Wednesday, despite longstanding doubts over the reliability of official figures.

More than 300 NBS staff have also been asked to return 3.23 million yuan ($500,000) they made by selling annual reports and providing data to affiliates, the Central Commission for Discipline Inspection (CCDI) -- the Communist party's internal corruption watchdog -- said on its website.

The news comes just months after the former head of the NBS was sacked in a corruption probe -- announced shortly after he appeared at a briefing on China's economy when he reiterated that the country's gross domestic product calculations were reliable.

Last year, official figures showed that China's economy grew at its slowest annual pace in a quarter of a century, but questions have repeatedly been raised about the accuracy of the Chinese government's economic data.

Critics say it can be subject to political manipulation in a country where the ruling party depends on growth for legitimacy.

The CCDI statement also said the statistics agency had failed to keep pace with changes in the economy, and noted problems with sales of handheld data-collection devices.