If you really do mean being 'laid off' i.e. not redundancy etc then, yes, it does have to be written into the contract to give the employer the power to do this otherwise it will be seen as constructive dismissal.
There are also guaranteed payments that still have to be made if a person is laid off, �19.60 per day up to a maximum of �98 in a 3 month period.
If the lay off lasts more than 4 weeks or more than 6 in a 24 week period then the employees can claim redundancy.
All this is if the employees have more than 12 months continuous service.
Hope this helps.