Forty-five drivers with valid Class 4 licences have been mobilised to help meet the shortfall of bus drivers at transport operator SMRT.

These relief drivers will be given a week of training to familiarise themselves with the routes and the buses before hitting the roads.

Minister of State for Transport Josephine Teo said on Saturday that it would take two to three months before SMRT will be able to recruit permanent drivers to replaced the 34 drivers the company has lost this week.

About 5 per cent of the company's bus routes will be affected and commuters may experience a slightly longer waiting time, added Mrs Teo.

The latest move comes after four SMRT bus drivers from China were charged on Thursday for instigating an illegal strike.

The Home Affairs Ministry said on Saturday that a fifth driver will be charged for a similar offence and that the police has also issued stern warnings to 29 other drivers, who have since had their work permits revoked, and will be repatriated by the Immigration & Checkpoints Authority (ICA) soon.