The Environmental Protection Department issued summonses to the corporation and contractor Leighton Asia. They face fines of up to HK$2 million and their directors up to six months' jail if convicted.

Representatives of both companies have been called to appear in Fanling Court on July 5 to face charges related to construction on a work area not covered by a permit issued by the department.

It could be one of the most serious offences committed by the corporation since work began last year on building the HK$60 billion railway that will connect Hong Kong with Panyu in Guangzhou and link the city to the mainland's high-speed railway network.

The MTR spokesman said: 'In hillside stabilisation works for the express rail link at Pat Heung in the last few months there is a technical misalignment in the relevant drawings and part of the licensed vegetation removal work may have encroached 20 metres on to the conservation area. The corporation has been in close dialogue with the relevant departments to discuss any remedial actions.'

One of the drawings the MTR Corp used had failed to show clearly the boundary of the conservation area, which had led to encroachment over an area measuring about 300 square metres, he said.

Trees could be replanted on both the work site and the cleared area after construction, as pledged in the environmental impact assessment study for the project, the spokesman said. None of the felled vegetation was on the government's list of old and valuable trees, he noted.

Leighton Asia, which was awarded a HK$3.2 billion contract by the MTR Corp for tunnelling and ventilation work in the Pat Heung area, could not be reached for comment yesterday.

A spokeswoman for the government department confirmed that the MTR Corp and Leighton Asia had been served with summonses for offences under the Environmental Impact Assessment Ordinance, but refused to disclose further details about the case. 'As the government has already instigated a criminal proceeding against them, it would not be appropriate to release further case details to the public,' she said.

A visit to the site by the South China Morning Post yesterday found construction work was continuing and some workers were still operating on the slope.

Dr Cheng Luk-ki, the head of research and conservation for Green Power, said the professionalism of the MTR Corp and its contractor would be cast into doubt if there were problems in the drawings. But he also suspects a common culture among frontline workers may be to blame.

'It could be a matter of ethics, as it is not uncommon for frontline staff to ignore drawings and do whatever they find convenient at a work site,' he said.

Cheng said the conservation area was a secondary forest where some wild animals could be found.

He said the removal of vegetation could lead to erosion and landslides. 'Should that happen, it would jeopardise a wider area of the conservation zone. So it is better for the MTR to replant as quickly as possible.'