The deal, signed with Guangdong province in 2002, set caps on sulphur dioxide, nitrogen oxides and soot emissions from power stations, factories and vehicle exhausts to be reached by 2010.

However, a report in the South China Morning Post English-language daily citing a 2005 Guangdong environmetal protection plan, said that Guangdong had unilaterally -- and secretly -- raised the emissions caps by as much as 158 percent.

The report said the Hong Kong government was aware of the change to the agreement.

Green groups said it raised serious doubts about the commitment of both governments to cutting worsening pollution in the region.

"Everyone will suffer as a result of this single-handed change to the targets," said Cheng Luk-ki of Hong Kong environmental protest group Green Power.

"It shows that Hong Kong is unable to influence Chinese policy and that Chinese economic growth is taking precedence over health and environmental protection," added Cheng.

Friends of the Earth Hong Kong said the revelations illustrated the lack of transparency in dealings with the Chinese authorities and accused leaders of "playing cat and mouse" with the environment.

"How can you put blind faith into an honour system agreement whose only enforcement is a promise, not a rule or law?" asked FOEHK director May Ng.

In a statement, the Hong Kong government said the recent figures from Guangdong were irrelevant to the emissions reduction agreement.

"Guangdong has its own purposes in preparing the plan. The plan does not stop Guangdong from achieving the emission reduction targets agreed with us," the Environmental Protection Department statement said.

"Both Hong Kong and Guangdong take the 2010 emissions reduction targets very seriously," it added.

Pollution has become a hot political issue in Hong Kong as smog levels have risen noticeably to often dangerous levels.

Poor air quality reduced visibility to less than one kilometer (about half a mile) on more than 50 days last year.

The government says the problem is mostly due to the industrialisation of southern China's nearby Pearl River Delta region.

Green groups blame local coal-burning power stations and diesel-powered buses.

Tour operators say visitors increasingly complain of pollution-related illnesses, while business leaders say the problem is driving away investors and executives.

Hong Kong's China-backed leader Donald Tsang has tried to make political capital out of the issue by meeting Guangdong environmental chiefs and launching air improvement initiatives.

However, environmentalists accuse the government of doing nothing to tackle the big-polluting power stations and factories in the region for fear of upsetting the powerful business groups that control them.

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