No contract of employment exists until a candidate has accepted an offer and all conditions under which the offer was made have been satisfied.
You can withdraw conditional job offers made subject to suitable references and criminal records checks, where the results are not as you expected.
If a candidate starts work before the results of checks have been received, you should make it clear that the offer may be withdrawn if the checks prove unsatisfactory. You may also wish to offer employment subject to a trial or probationary period. The length of the period may depend on the type of job and how much time is needed to demonstrate the necessary skills.
If you decide to withdraw the offer at the end of the period, you need to give the employee the notice period specified in their written statement. It's also advisable to explain clearly why the offer is being withdrawn to avoid potential legal claims, e.g. for discrimination.
If no notice period has been agreed, they are entitled to the statutory minimum notice period, or to any longer period which is the established custom or practice within the industry.
An alternative to withdrawing an offer is to extend the probationary period - if the contract allows - and to provide appropriate training.
Employees cannot claim unfair dismissal before completing one year's service (if employed before 6 April 2012) or two years' service (if employed on or after 6 April 2012), unless it is for a number of automatically unfair reasons.
However, an employee dismissed during - or at - the completion of their probationary period may be able to claim breach of contract if - for example - you have not provided training that you promised would be given.