It said it would follow standing procedures to appoint a new director and make an announcement in due course.

The director of immigration is one of the principal officials whose appointment and removal has to be approved by Beijing under the Basic Law.

Immigration Staff Association chairman Wong Tong-sing said he was 'a little shocked' to learn that Mr Lai was said to be leaving in April.

'We had heard earlier that his retirement was due by the end of this year,' he said.

But he said it was not unusual for senior officers to opt for pre-retirement leave rather than staying until the last minute.

'Actually, the director post is a tough one. It's a very sensitive post and the director faces a lot of pressure.

'I don't see anything particularly unusual. It's a matter of personal choice. It's not really early retirement,' he said.

The staff association hoped that the post would be filled internally, saying deputy director Simon Peh Yun-lu was an ideal choice, Mr Wong said.

'We have established good relations with Mr Peh. Internal promotion would enhance staff morale and the development of the department in future.'

He said he was not aware of any plan by the government to appoint an administrative officer to replace Mr Lai.

The government source also said the top post was likely to be filled by the deputy instead of an outsider.

The source said officials could choose to take accumulated leave before their official retirement date.

Mr Lai joined the Immigration Department as assistant immigration officer in 1973 and was appointed director in 2002.