Union officials and management are due to meet today to discuss the redundancy plan, which is expected to be carried out over the next few weeks.
Several hundred of the 4,000 staff are likely to be made redundant and unions fear that they will be offered terms less generous than those available under the old BR agreement, which are more than double the statutory minimum.
South West Trains admitted last month, shortly after the takeover by Stagecoach, that 125 white collar staff were being made redundant, including a quarter of senior managers, in a first round of cuts. It warned then that further reductions in the workforce were inevitable.
South West Trains is under pressure to make sharp cuts in expenditure because the franchise was won by Stagecoach with a very low bid.
In BR's last year of management, the line received a subsidy of pounds 63.5m, but Stagecoach is due to receive only pounds 54.7m in the forthcoming financial year.
Yesterday, Peter Field, managing director of South West Trains, warned on BBC Radio 4's Today that a restructuring of the company was planned and said: "Several hundred jobs will go at the end of the day."
Job losses are likely to be targeted most severely at the 400 headquarters staff as layers of management are removed, a standard Stagecoach approach when taking over companies. They are also expected to hit the 2,000 staff employed to look after stations, sell tickets and see off trains.
The company is likely to avoid trying to reduce the number of drivers because of their industrial muscle.
Mr Field promised that no station would be destaffed by these latest cuts and that there would be no reduction in the ticket office opening hours.
However, the cuts were seized upon by Labour as showing that privatisation will lead to massive job losses.
Clare Short, Labour's shadow transport secretary, said: "There is no reason to suspect that these type of job cuts will not be repeated by the other operators.There will be an adverse effect on services, with fewer staff at stations and greater pressure on the health and safety of both staff and the public."