Can an employer make permanent employees redundant while there are agency staff carrying out similar roles?

If an employer makes permanent employees redundant while retaining agency staff to carry out the same or similar work, it raises the question of whether or not the redundancies are genuine or necessary. The employer would have to be able to show that the business requires fewer people to carry out the work, which would be difficult to argue if it continues to use agency workers. If the employer can show that agency staff are used only in specific contexts, for example where there is a limited requirement for particular work to be carried out in the short term, and that permanent employees are not required in those roles under the long-term structure of the business, it may be able to show that any redundancy dismissals were fair.