What should I put in a complaint letter?

A complaint letter is not simply a way of expressing dissatisfaction. In every regulated sector it is the document that triggers a process with legal obligations attached, and it later becomes the first exhibit in your ombudsman file. It has three jobs: it starts the clock that produces a final response or an 8-week right to escalate, it fixes the facts while they are fresh, and it defines the remedy an investigator will later be asked to award.

The shape that works

1. Your details — name, address, email, phone, and the account, policy or matter reference.
2. Subject line — "Formal Complaint — [product or service] — [account number]", so it is logged as a complaint rather than a query.
3. Opening — one sentence saying this is a formal complaint and what it is about.
4. Chronology — dated bullet points, not prose: "12 March — reported fault by phone, ref 4471. 19 March — engineer failed to attend."
5. The breach — the contract clause, published service standard, regulatory rule or statutory duty you say was not met.
6. Impact — money lost, time spent and practical consequences, quantified separately.
7. Remedy — exactly what you want, in figures.
8. Deadline and next step — a date, and the ombudsman or regulator you will go to if it passes.
9. Enclosures — a numbered list of the documents attached.

Wording that helps, and wording that costs you

Phrases that change how a complaint is routed internally include: "I am making a formal complaint and ask that it be logged under your complaints procedure"; "please treat this as a request for a final response in accordance with your regulatory obligations"; and "if I have not received a satisfactory response by [date], or in any event after 8 weeks, I will refer this complaint to [named ombudsman]". Asking for the allocated reference number is worth a line of its own.

What costs you is threats you will not carry out, abuse, and sheer volume — twelve emails in a week buries the one that mattered and gives the organisation grounds to restrict contact. Send it through the published complaints channel rather than general customer service or social media, where it often never enters the complaints system and the clock never starts. Email is usually best: date-stamped, searchable, and easy to forward to an ombudsman later.