A LACK of manpower has forced the Immigration Department to put off plans to form a 100-strong task force to battle the increasing problem of illegal immigrants. The Government has decided to postpone the move for at least another year, claiming there was not the manpower available to set up the force. While it is expected that the department's workload will increase due to a boom in cross-border traffic, 33 posts still have to be deleted from the 5,655-strong establishment under government budgetary constraints. Total expenditure for the department in the coming financial year is estimated to be $1.12 billion, which is only half a per cent higher than this year. Principal Assistant Secretary for Security, Mr Simon Vickers, said yesterday it was unlikely the department would get extra funding to establish the task force this year, which would have to recruit extra staff. The annual 15 per cent to 20 per cent increase in passenger traffic across the border had also put great pressure on the need to deploy more staff at various control points, Mr Vickers said. ''The proposal has not been abandoned, but we have not got the funding this year to set up the force. We will consider it again next year,'' he said. The idea of the task force was first proposed by the Director of Immigration, Mr Laurence Leung Ming-yin last May. It was intended to be an operational unit to carry out raids in search of illegal immigrants and gather intelligence. It would also help cope with the increase in passenger traffic at control points during holidays. About 7,600 illegal immigrants from the mainland have been arrested so far this year, an increase of almost 50 per cent over the same period last year. Assistant Director of Immigration Mr Cheuk Koon-cham said the decision would not affect their anti-illegal immigrant operations, but they would not be able to spare extra staff to conduct more extensive investigations. Because there was a constant shortage of staff at Lowu control point, the Government hoped to first solve this problem, he said. According to statistics, a total of 34.9 million passengers passed through Lowu last year, an increase of more than 10 per cent over 1991. The number is expected to grow at the same rate this year.