A warning or threat of disciplinary action from your HR department should never be the first indication you receive that your behaviour or performance is under question.
The first step you should take is therefore to read the letter from HR carefully and identify the accusation being made against you. If this is something you’re already aware of, you’ve discussed it with your manager and you’ve failed to improve, then your employer may be right to escalate the matter.
Then, before you take any further action, you should seek advice from a union representative or a professional advice service. By obtaining expert advice, you’ll be able to protect your rights against unfair dismissal.