'To be frank, our management and legal supervision of exported labour is not very comprehensive,' said Mr Kong.

He said the alleged wage deductions had concerned both central and provincial leaders.

But China's labour authorities could do little to help as labour service companies were not under their supervision, Mr Kong said.

'Our job is to supervise the implementation of Labour Law to safeguard the legal rights of our citizens including those workers exported,' he said.

'If we find a labour service company charging rampantly, we must make it pay back the excessive amount to workers,' he added.

He said the mainland authorities would consider providing help to Hong Kong over such incidents if the territory made a request.

But Mr Kong added it was up to the Hong Kong authorities to resolve the problems faced by mainland workers.

It would be impossible for Chinese officials to visit the territory to deal with individual cases, he said.

The labour official backed the idea of giving the Hong Kong Government a list of authorised mainland labour service companies to help future approval of mainland workers.

'I think that should be done,' he said.

'As the foreign economic and trade authorities take charge of labour exports, such a practice will be considered,' he added.

Meanwhile, Guangzhou Mayor Li Ziliu admitted there were loopholes in managing labour service companies and intermediate organisations in the city.

Mr Li said Guangzhou would strengthen its supervision of such companies and would also check their legality.