Do I have to tell my employer about my diagnosis?

No. Under the Equality Act 2010 nobody has to tell their employer they are disabled or share a diagnosis. But your employer only has to make reasonable adjustments once they know about it. So staying quiet is your right, and it is also usually what switches the duty to help off.

What the law actually says

Acas is blunt about it: by law, nobody has to tell their employer they are disabled. The same goes for naming a specific condition. You can go through recruitment, and your whole working life, without ever disclosing an autism, ADHD or other diagnosis, and you keep your job either way. The National Autistic Society says the same thing for autistic workers in plain terms: no, you do not have to tell an employer you are autistic, in recruitment or once you are in post.

The catch the headline answers miss

Here is the part most search results skip. Your employer is under a duty to make reasonable adjustments for a disabled worker, but that duty only starts once they know, or could reasonably be expected to know, that you are disabled. If they have no idea, they are not required to adjust anything, because they could not reasonably be expected to. Much of your protection from discrimination works the same way. So the trade is real:

• Tell them, and the duties switch on. They have to consider adjustments such as flexible hours, written instructions or a quieter workspace, and treating you badly for something connected to your condition becomes unlawful.
• Say nothing, and you keep your privacy but lose the practical route to adjustments, since the law does not expect an employer to act on something they have no reason to know.

Two things worth knowing before you decide

First, the protection rests on the legal definition, not the label. A person is disabled under the Act if they have a physical or mental impairment with a substantial and long-term effect on their normal day-to-day activities (Equality Act 2010, section 6). You do not need a formal diagnosis to meet that test, and having one does not guarantee you do. Second, telling your employer does not have to mean handing over your diagnosis. You can disclose the support or adjustments you need without naming the condition, and you can route it confidentially through occupational health or HR rather than your line manager.

This page is general information, not clinical or legal advice.