The move, which may be announced in next month's policy address, follows a government study that concluded keeping such a large area closed was now hard to justify. The study also found that security would not be jeopardised as the problem of illegal immigrants - which the closed area was originally designed to combat - was largely under control.

A government source said about 800 hectares, including the land around the Sha Tau Kok, Lok Ma Chau and Lowu border crossings, would remain closed. The divided town of Sha Tau Kok would also stay closed, despite calls from district councillors and lawmakers for it to be opened, at least partly.

Heung Yee Kuk chairman Lau Wong-fat said he had discussed opening the border zone with Donald Tsang Yam-kuen when kuk representatives met the chief executive on Wednesday.

'I believe [he] will mention how to develop the border zones when he delivers his first policy address next month,' he said.

The news could set off a new battle between conservationists - emboldened by their success in preventing the Kowloon-Canton Railway Corporation from building a viaduct across the Long Valley wetlands - and developers and villagers wanting the best value for their land.

The source said it was unlikely that any new towns or big developments would be allowed in the area. Most of the agricultural area would be kept, and sites with high conservation value would still be protected. Village expansion and some low-rise development would be permitted.

'The prevailing view is to maintain the status quo of the zoning of most of the land,' the source said, adding the changes would promote ecotourism.

The closed area was established in the early 1950s as a buffer against a huge influx of people fleeing turmoil on the mainland.

But the government source said: 'Mainlanders nowadays do not sneak into Hong Kong. They cross the border by using two-way permits or the individual travellers' scheme.'

Roger Tang Man-hung, former vice-president of the Hong Kong Institute of Planners, said the ecosystem at the border would be severely damaged, if not destroyed, by human intrusion.

'After being closed to the public for so many years, the land has established its own ecosystem,' he said. He also warned that many so-called ecotourism projects were destructive. 'In the mind of the government, they think ecotourism is conservation. This is wrong.'

The future of the Lok Ma Chau Loop - the no-man's land on the border formed by realignment of the Shenzhen River for flood protection - was not part of the study, the source said.

Hong Kong and Shenzhen officials have already agreed to set up a group to study the feasibility of developing the 100-hectare zone.