How do you write a strongly worded letter of complaint?

If you’ve ever walked away from a poor service experience, product failure or unresolved issue thinking, "I should write a complaint," this guide is for you. But not just any complaint, a well-structured, calm, and strongly worded letter that gets read, taken seriously, and prompts action.

At LetterLab, we help clients write impactful complaint letters every week, from minor annoyances to major oversights. Let’s look at how to write one that hits the mark.

Why write a formal complaint letter?

Writing a complaint letter isn’t about venting. It’s about making a case. Done well, it:

• Establishes a clear record of your issue

• Encourages accountability and resolution

• Helps you stay professional and persuasive

  
  

According to Citizens Advice, a written complaint can be more effective than a phone call because it creates a paper trail and allows you to express your points clearly.

Step 1: Start with the facts

Open with the most important details:

• What happened

• When and where

• Who was involved (if applicable)

• A brief summary of the problem

  
  

Avoid emotion here, just lay out the bones of the issue so the recipient can grasp it quickly.

Step 2: Explain the impact

Now make it human. How has this issue affected you?

• Were you financially out of pocket?

• Was your time wasted?

• Did it cause inconvenience, stress or embarrassment?

  
  

You’re not moaning, you’re demonstrating why the complaint matters. A sentence like, "Due to the late delivery, I missed an important client presentation and lost business," is perfectly reasonable.

Step 3: Show what you’ve already tried

Have you emailed? Called? Spoken to someone in store?

Outline what you’ve done to resolve things. This shows you’ve acted in good faith and aren’t jumping to a formal complaint unnecessarily.

According to Which?, keeping a record of who you’ve spoken to and when can support your case and help escalate it if needed.

Step 4: Clearly state what you want

Do you want:

• A refund?

• An apology?

• A replacement?

• Compensation?

  
  

Be specific and reasonable. A vague, "I hope you do something about this" isn’t helpful. Try, "I would like a full refund of £89.99 within 14 days."

Step 5: Keep it polite, but firm

This is the hardest bit. You're angry. But keep it professional.

Some tips:

• Don’t use insults or sarcasm

• Avoid ALL CAPS (it reads like shouting)

• Use phrases like "I am disappointed..." or "I expected better from your company"

• Let your reasoning, not rage, do the heavy lifting

  
  

As the Financial Ombudsman Service notes, a clear and courteous complaint is more likely to get a timely and fair response.

Sample structure to follow

Here’s a basic layout to help:

1. Opening: "I am writing to formally complain about..."

2. Facts: What happened, when, where

3. Impact: The inconvenience caused

4. Previous steps: What you’ve already tried

5. Resolution: What you want to happen next

6. Close: "I expect a response within [reasonable timeframe]"

   
   

Real-life example from LetterLab

One of our clients recently ordered a bespoke item online, only to receive something completely different. Emails to customer service went unanswered. We helped her write a clear, well-paced complaint asking for a refund and return postage. She received a full refund and apology within a week.

No shouting. No drama. Just results.

Final thoughts

A strongly worded letter of complaint isn’t about aggression, it’s about clarity, structure, and tone. It puts pressure where it’s due, while keeping your side of the street clean.

And if you’re not sure how to start or want it worded just right, LetterLab can help. We’re the UK’s go-to letter writing service for everything from complaint letters to character references. Let us put your case forward with precision and power.