COMPLAINT LETTER GUIDE
How to write a formal complaint letter that is easy to investigate
A practical structure for a formal complaint letter: clear facts, chronology, evidence references, requested outcome and a professional close.
Reviewed 9 September 2026 · UK guidance
QUICK ANSWER
What should you do next?
A useful complaint letter identifies the customer and transaction, states the central problem in a few lines, sets out the key events in date order, refers to supporting evidence and asks for a specific outcome. It should be factual, proportionate and easy for someone unfamiliar with the case to follow.
Use a clear opening
Put the words “Formal complaint” in the subject line. Give the organisation enough information to locate the account, order or booking, but do not include passwords, full card details or unnecessary sensitive information.
- Your name and safe contact details
- Account, order or complaint reference
- Product or service involved
- Date the problem began
- One-sentence summary of the complaint
Separate facts from conclusions
Describe what happened in date order and distinguish what you know from what you believe. Quote important promises accurately. If a telephone conversation matters, identify the date, who took part and your note of what was said.
- Short dated events
- Names or departments where known
- Exact amounts
- References to numbered evidence
- The company’s response so far
Ask for a specific resolution
Explain what would put the matter right: for example, correction of an account, completion of a service, a refund of a stated amount, reconsideration of a decision or a written explanation. Avoid demanding an outcome that the recipient cannot provide.
- State each requested action
- Show how any loss figure was calculated
- Ask when you can expect a response
- Request escalation details if unresolved
Edit before sending
Remove repetition, insults, speculation and unrelated history. Check every date, name, amount and attachment reference against the original evidence. Keep the full version you send and proof of delivery.
BEFORE YOU ESCALATE
Complaint pack checklist
- ✓Formal complaint identified
- ✓Reference details included
- ✓Main issue summarised
- ✓Events in date order
- ✓Evidence numbered
- ✓Outcome clearly stated
- ✓Final accuracy check completed
Common questions
How long should a complaint letter be?
There is no ideal length. It should contain the information needed to understand and investigate the issue without repeating the same point.
Should I sound legal or aggressive?
No. Plain, accurate language is normally easier to investigate. Use legal wording only if it is relevant and correct.
What does ProofPackAI add beyond a letter?
It can organise a cover letter, chronology, loss schedule, case summary and indexed evidence alongside the complaint itself.
Official information to check
Processes and time limits can change. Confirm the position for your complaint using the relevant official service: