References are usually sought from current and former employers.
Most employers will ask for references to be submitted in writing, either in the form of an open unstructured letter or in a standard form.
All data given in a reference should generally be based on fact or capable of independent verification. Referees should be very cautious about giving any subjective opinion (which cannot be substantiated with factual evidence) about an individual's performance, conduct or suitability.
In law there is nothing to prevent a prospective employer asking if you have ever been subject to disciplinary action or an investigation. If you would like guidance on how to approach this issue with potential employers, contact us.
You may also wish to look at the ACAS guidance 'Providing a job reference'.